Hmm.. well, I don't know if I quite qualify as a good mentor, but let's see what I can do.
Every post is an essay. This sounds bizarre on it's front, but I've found that I approach posts much like I would an essay for a class. One of the greatest strengths of the PbP medium over F2F is the fact that you can run a monologue or detailed description much longer than you can normally. If I was to say what I type, it'd probably leave me hoarse and my players asleep. Or deaf. Or both. At the same time, you'll almost be expected to type more than you'd say- a post of "Bob swings at the orc again," is going to be a dull read- and possibly come off as if you're really not interested in the game. Especially as a GM, this is important! You're describing the game world and you're pulling your players into it.
Allow me to give an example from my game. You could simply say, "You walk into the curiosity shop. Andrei greets you as he finishes lighting candles," Or you could say this:
(Griffin Marlique - City of Ghosts, Andrei's Curiosities, Malleus 7th, Cinten 604 AR, Twilight)
The small bell over the door rings as it's pushed open, yeilding fitfully as it hits the metal curl that sets the bell in motion. The place is old, darkened wood- there's as much cigar smoke as stain in the cause. Tables divided up into shelves form haphazard rows and corridors through the shop. The smell of incense- Andrei's idea for making the shop seem cozy- thickens the air, leaving it dark and muddied in fading light. By now the rain has started full force, and guttering lamplight tries to fill the store, gathered from dozens of small lamps arranged haphazardly among the merchandise on shelves and tables. For all that can be determined, some of the lamps might well be merchandise. The front of the store is filled with simple bric a brac, wooden and rope toys, old drawers set aside and filled with buttons, fake gems made of glue, and various similar things. As they move towards the counter in the center of the store things polymorph into something more valuable- tools, secondhand, toys of fur and cloth, clockwork toys, pocketwatches, bolts of cloth in unusual patterns, and sculptures made from porcelain and ivory. As one moves past to the back of the store, bookshelves line the walls, leaving space for strange unguents and components of dubious alchemical value, old copies of newsprint, and a half-dozen stuffed beasts- a bear, a falcon, a moorcat, a raccoon, a dragonfish, and what Andrei claims is the preserved foot of a Skorne warbeast.
The counter itself is wooden, with thick glass panes revealing all sorts of expensive items- bejeweled knives, jewelry, swords, a gauntlet, and small mechanikal components- again of dubious value. The man himself is moving about the store, an older fellow of uncertain heritage, perhaps Morrdh, perhaps something else, his hair is grey and ringed around his bald head, his eyes hidden behind thick glass spectacles (that are purely for show), a bulbous nose and a thick lantern jaw that remains all but free of a beard. He moves among the last few lanterns, lighting each in turn as he does so, lending a yellow cast to his pale skin. "Ah? Ah! Yes. Griffin. I will be with you in a moment. I expect you are here for business? Not, I think, for a mug of ale and old stories, which I shall keep as well as any other of my curiosities."
90% of the post serves no other use than to tell the player a whole lot of things he doesn't need to know- but at the same time it provides the flavor and description that brings the world alive. Plus, it'll get people to put as much effort into their replies as you do.
Now- on the opposite side, don't go on for eight pages about a swing of a sword. There is a point where your players' eyes will glaze over and nothing will get through. I personally strive for about three paragraphs, unless there's just not enough to work with.
Corrollaries to the above: Feel free to have your NPCs spout mini-monologues- if they're going to greet someone, have them also comment on the weather and make observations about the PC, not just say, "Hi how're you?" Pick out details in weapons, describe the lighting, smells, and sounds. If you write the same thing for two people at two different times, vary how you say it, don't just cut & paste.
Nothing is written in stone until you hit Post. Reread your posts, see how they sound, make sure you got everything you wanted to say. If you're halfway through a plot and you come up with a better way of doing things, feel free to change it- just so long as you aren't creating continuity problems. Always know how much of the plot thread is still loose, and feel free to manipulate it to make things more enjoyable. Just make sure it has an end, too- a lot of people I know are giving up on Lost because they're convinced the "It all has a reasonable explanation," thing is just so much hooey, and there won't be a real ending.
Intrigue is more fun than combat. Sort of a corollary to the essay thing- combat tends to involve short time segments where everyone's waiting for someone else to post. There's a lot of repetitive action, and especially in systems like d20, it can take seemingly forever to wrap up a big fight. Don't be afraid to have the occasional battle, but try to steer towards mysteries and intrigue. PbP is a much better medium for that than fighting things. People have time to stop and think, mull things over. They read and think, and get to use their minds.
More after I get home- I love these days at work where my job's to answer a silent phone.
