Or "DM the way you will enjoy, instead of the way that appeals to as many people as possible."
This list is not presented in a logical order, but the order in which they popped into my head.
DM Small Groups Only Edit
4-6 players is ideal. 4 players + DM allows for use of Dungeons as a setting. More players makes everything chaotic and the quality suffers for everyone involved.
Events Don't Have To End In Camp Edit
Having to plan every event to get all players safely home in time for supper (and casual RP) is very restrictive. You can't pose a challenge to players that you aren't sure they can solve within a few hours and so the stakes only ever get so high.
CASUAL RP IS NOT GUARANTEED - sometimes the event might end with the situation unresolved, leaving your characters in limbo (and IC time paused) until the next event.
Individual Player Turns ONLY Edit
Group turns suck. Players can't react to each other strategically, no one reads each other's emotes because they're too busy writing their own, and as a result no character ever really gets a moment to shine unless they get a critical role that draws attention. Group turns also ruin certain mechanics.
Individual tuns gives each person a chance for the spotlight, and the players who are going after them can react strategically to the changing of events.
Monsters can be added to the turn order BETWEEN players instead of only after EVERY player has acted, meaning that the first group of enemies in any given combat won't always be summarily wiped out before they can even react.
All Character Abilities Must Be Declared Upfront Edit
A character suddenly debuting a brand new spell or ability that just so happens to completely counter the current challenge is rarely fun for anyone else. If the DM doesn't know your full list of abilities, they cannot plan for it.
Only the DM must be made aware of your characters full power - you can of course keep things secret from other players to surprise them. (And this is encouraged!)
All Character Gear Must Be Justified Edit
Similarly to above, the things your characters own (weapons, armor, equipment) must be declared and justified as why they have them. This helps ensure a consistent internal "lore" about the type, quality, and rarity of objects in the world and the relative power-levels of characters in the RP.
Changes In Gear MUST Be Explained Edit
Suddenly appearing a whole new outfit while you're camped out in the middle of a jungle can raise a few eyebrows. But don't just assume because you're in a town that you have access to all the amenities you want - ask the DM if there is somewhere you can obtain <gear that you want.>
If not, you'll have to find (or make) an opportunity to go someplace that has it.
All Special/Magical Gear Must Be Obtained Through RP Edit
When a character suddenly shows up with a giant pair of glowing swords with little or no explanation, it can be very jarring for RP. What's more, the DM has lost a chance to present you with a cool piece of magical loot to justify your new shiny transmog.
Special and Magical items are awesome, and they deserve lore to their existence and a story as to how you obtained them! If you REALLY need that specific transmog RIGHT NOW, you can discuss with your DM a way to obtain it that makes sense.
Apply Death Rules To All Players Edit
Whether death is enforced or characters cannot die, all players need to abide the same rules. A disparity in which characters can or cannot die make it difficult to present all players with an equal sense of tension.
(Enforced death is preferred, the tension it creates cannot truly be replicated by anything else.)
Your Character Can (And Should) Act Off-Screen Edit
As well as being good for keeping up immersion and consistency in RP, off-screen time can be useful for your character to accomplish important things that might be mundane to RP out in detail.
Whenever there is a passage of time IC with RPing every moment (such as travelling, resting, waiting, etc.) you are encouraged to declare your characters actions during this time. This can be trivial things for flavour or things that actually benefit your character in some way. Any necessary roles can be made all at once when the action is declared, and DM tell you the results of your action.
Your Characters' Personal Lives Matter Edit
Whether a long term goal they've been working on, or an arch-nemesis looking for revenge, there are many things about your characters personal life that can greatly enhance the RP for yourself and others, and make you more invested than ever in how events turn out.
Tell the DM about anything you're willing to give them discretion to introduce into the story. Not being in control of every facet of your characters life (especially their enemies) is great fun!
OOC Absences Need An IC Explanation Edit
Missing events cannot be helped, but characters vanishing and re-appearing on a whim to suit a players OOC needs is very disruptive to RP.
If you need to skip an event, give a reason (or ask the DM to provide a reason) for your character to be ICly absent so that things make sense. If a reason cannot be provided, your character will be considered to be present and if interacted with the DM will provide their response.
For important actions (such as combat) the DM should endeavour to excuse your character instead of acting on your behalf, but this may not always be possible.
Absences After A Cliffhanger Edit
In the event that the previous session ended in a cliffhanger (such as during or just before combat), any player who cannot attend the resolution will have their character DM controlled until the immediate situation has passed (such as the end of combat, or arrival at a safe location.)
The DM will then try to excuse your character as per the above rule.
A player not being able to control their own character is unfortunate, but it is preferable to a character vanishing in the middle of important events due to OOC scheduling problems.
(Practically speaking, this rule should mostly be avoided by scheduling only when all players are available, but this is not always possible in practice.)
Never Trivialise Travel Edit
Long journeys can be dull, and since the purpose of RP is to have fun, it is often in everyone's best interest to skip over (with a brief summary by the DM) any mundane task such as a long and uneventful journey.
However this can sometimes have the side effect of bypassing what might otherwise be a challenge in the journey for the sake of brevity or convenience. This is bad for the sense of realism and consistency in the world.
As a compromise, during the summary of a journey a DM might throw a small challenge at the players; e.g. "You reach a river with no obvious signs of crossing. What do you do?"
In response the players can provide a simple answer "We look for a safe place to cross", "We try to swim", "I use my magic to FREEZE THE RIVER AND MAKE AN ICE PATH BECAUSE MAGES COUNTER EVERY CHALLENGE", etc.
The DM can then ask for some quick rolls and move on to the more important parts of the RP. This helps keep the journey RP brief and to-the-point while still doing the world justice.
If the responses to such a challenge become overly complex, the DM can easily just drop everyone back into full-RP mode to resolve.
Limit Usage of OP/Challenge-Nullifying Abilities Edit
Some abilities possessed by certain characters - mostly mages - can often be frustrating for a DM as they can completely bypass a lot of interesting challenges.
The most common problem is portals - all travel concerns can be nullified if the party has a portal-capable mage to port them all over the kingdom. Similar problems can encountered with abilities like flight, invisibility, etc.
Blizzard in general do a very poor job of detailing the restrictions of high end magics like portals. This is often because they themselves like to trivialise the logistical problems of sending an invading force to a foreign world only to have their only means of entry destroyed, leaving them stranded. A quick portal will fix that right up!
Identify troublesome abilities and decide upon reasonable restrictions to them upfront, and make everyone aware. (For example, a portal could require both the starting location and the destination to be prepared beforehand, before a portal can be opened.)
Do Not DM And ICly Lead At The Same Time Edit
As a DM, you should only be giving your players the information they need to progress on their quest. You are a quest giver giving them objectives. You should not be the compass guiding them to the exact location they need to find, or dropping the NPC they need to talk to right in their lap.
Player agency is important, and if you are guiding players from one set of objectives to the next and only asking for their input when they need to roll a skill-check, you are removing an important part of the experience.
As a DM, you should be the Police Chief telling your officers "Go out there and find that criminal scum!", not one of the Police Officers saying "I think we should go talk to Suspect A."
The easiest way to achieve this is to simply not roleplay a character with any authority - or even any character in the party at all. You will have plenty of chances to roleplay a variety of characters that your players meet along journey.
Hint your players towards the objectives you want them to find, but allow them to succeed (or fail) on their own. React to their decisions and let the story go where it may.
Remember, as the DM you are roleplaying an entire world, bringing it to life as your players explore it. They have free will, and its your job to guide them towards the challenges you want them to find, not railroad them.
ALL ROLEPLAY SHOULD HAPPEN WITH THE DM PRESENT Edit
While this is a tough one as it limits players ability to casually RP during down-time at peaceful times, the DM needs to be present for any and all RP that occurs - even casual RP.
The DMs job is to bring the world to life, and provide context and consistency to everything that happens. They ARE the world. Decisions that may seem trivial still need to be made by the DM.
You might be casually chatting and decide that its reasonable for there to be some wine in your camp supplies. At the next event, a travelling drunkard stops by your camp desperate for a drink, and offers a magical trinket if he can just have one drink of wine. You go to fetch some, and the DM informs you that you do not have any wine among your supplies.
This creates an inconsistency because you did not know what the DM had planned, or that wine could possibly be important. You didn't know that the DM had planned for a social encounter with a nearby group of campers who DO own wine, with the objective being to bargain for the wine from them without tipping them off that they could get a better deal if they bypass you and deal with the drunkard themselves.
This is a trivial and extremely unlikely example, and a good DM can easily cover the inconsistency by just adding the detail that "you did have wine, but you drank it all last night." Nevertheless, the point remains that you don't know what you don't know about the world - or even your own character:
Your party arrives in a new city and travels through it looking for an inn to stay in. During the trip through the city, the DM makes some unexplained rolls and then asks your character for a Perception check. You roll low, and the DM simply says "Okay" and does not explain the rolls.
After the session, you have some casual RP with another party member where you buy a drink at the inn you are staying at. You don't ask the DM to oversee this as it seems like such a trivial thing.
Had the DM been present, however, he would have told you that when you went to pay for your drinks, you would find your coin pouch empty. You see, it turns out that the Perception check that you failed earlier in the night was to notice a Cut-purse stealing your gold, and you are now as penniless as the beggars you ignored on your way here.
The lesson is simple; all RP should occur with the DM watching, because we are always keeping secrets from you.