Keep your plots short and simple, especially at first.  Wrapping things up in one session or over no more than a week with a small group of players is ideal.  It helps avoid us getting spammed with avatarmail or questions when you aren't around or having it get out of control and too many people involved.  Other immortals are busy with their own stuff and simply keeping up with day to day issues, so try to avoid adding to the load on others unless you decide in advance to work together on something.

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Focus a lot of your interactions on the lowbies/mids, especially when you are starting out.  They are easier to guide, have less of their own thing going on, and we need to get more new players hooked early on RP.  Regardless of whether they are alts or new players, they need to be either nudged into character development/RP or have more opportunity to express and use what they have developed in their lower levels.
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Perhaps make some personas in the lowbie/mid range to act like PCs and adventure with them.  Give them a rich personality, appropriate descriptions, and let them have weaknesses including managing with less than max gear to set an example.  Note:  don't go overboard with exotics like halflings, kobolds, children, etc.  Make it a solid adventuring character that demonstrates what we want players to play.  

Use the persona or orb NPCs to interract with PCs about why they are doing what they are doing.  Make them think about and verbalize their alignment, beliefs, gods, racial attitudes, goals, background, etc.  Teach them languages, RPing spell research, how to be a cleric and what a given god/dess stands for, etc.

Better to give small simple rewards to many and more often than big things to just one or two people.  Alterobj is quick and easy.  Just be careful not to totally change something that is special, like the dresses from the valley that have special emotes, etc.

Try to be careful to not start something with people that are already involved in something else or that will spill over.  HMs are often doing RP stimulating things on their own, so don't overload them by dragging them into something else without watching to be sure it's not going to be too much of a distraction for the time they have or take away from another plot or things they need to do to support other PCs.  

We've also often had PCs think something they heard that one avatar started was somehow related to an entirely different plot.  Pretty soon people are frustrated and confused because there is too much going on and changing day to day to have a prayer of keeping up if they aren't here 24/7, which most can't be.
  
Make sure we don't create no win situations.  The players should always have a "way out" even if it is daunting or means they have to get creative.

Logging in and not being able to figure out what to do either from having too big a list or lack of direction is something I think burns out a lot of immortals.  Watching for abuse is helpful but will probably burn you out fast if you aren't careful.  It's tempting to just hide out, watch, pass judgement, make notes about or yell at people who screw up, etc. but there are many other ways to nudge players to improve that are far more fun.

A downside of avatars is when you guys are on and invisible, it means a good RPer is out of the game.  So, especially when player count is low, it's important to have a persona ready to log in and help fill the gaps, especially to give new players someone to interact with.  You can also use them to distract someone who is out cycling and see if they will stop to interract and how well they do.

When making a persona, pick a PC you see on that's comparable to the level you're trying to emulate and make your stats, hp, gear balanced to that.  Many/most of your personas should look, behave, and talk like just a regular PC/adventurer.  They don't all need to be special occasion/conflict/plot oriented.

Some of the complaints we get about people who seldom ("never") go adventuring are because those people are perpetually in the middle of plots, RP, and other conflicts.  They simply don't have the time after they keep in touch with everyone they need to about everything.  That creates double negatives.  Try to put more of the "burden" for carrying plots onto the power gamers and PLers to give them something else to do and make sure they don't dump it over onto the RPers or it gets taken over by the others.

The ones who focus on adventurering often dislike the ones who RP more than adventure and avoid them because they can't get them out exploring, i.e. the finger pointing about "all so and so does is sit around and chat" and gives RP a bad rap.  It gives the ones who struggle with character development and prefer hack and slash even more excuses to distance themselves from RP, especially if they don't have time to keep up with long plots.  So, reward the "RPers" so they know you noticed their good RP by supporting what they are doing, but not by sticking them with new twists and challenges all the time.  More importantly, help them get away from the soap opera stuff.  Give them reasons to gather parties now and then somehow to go explore an area that they might not have been rather than distract them with plots that require a lot of communication, strategizing, etc.

see also:  new-imm, imm-rules, guidelines, builder, avatar101b