1. Don't go too big. More players means more work. You can always add players later a couple at a time if you think you can handle them - if your game is good then they'll hop on pretty quickly.
2. Don't feel like you need to prepare for everything. You're on a forum. You can take a day or two to deal with confusing situations, so don't pack your OP with every little detail of how the game is going to work. You'll just end up bogging yourself down and confusing your players.
3. Plan ahead. It looks like you're doing this. Of course, this isn't completely necessary, but the less of this you do, the more you'll have to...
4. Be flexible. If a player is doing something unexpected, take a little time to think up a good response. Not everything deserves one, but don't shut down your players every single time they try to do something outside of the box. Those kinds of players are the kind that will take you places you wouldn't go on your own.
5. Take advantage of the forum format. This might just be a little expansion to 2, but I think it's worth saying. If a player surprises you, you're not there in the room with them. You can pretend not to be surprised. You can think up a response that makes it look like you were planning for that all along. This can greatly increase immersion and reward creative players as much as you're rewarding the players who stick to the main story.
See, don't you feel clever?
The forum format also has some minor cool stuff like invisible text (highlight the line above this) that you can fiddle with for fun times. I'm not really sure how much you could take advantage of that sort of thing, but creativity is awesome and I bet someone could do some really cool stuff with it.
3b. Have your OP fully built and ready to go in a text file. Have your basic ruleset and character sheet prepared. Have at least the plot and encounters of the first mission/first dozen or so full updates before you post the thread. This is all a tremendous help in getting it off the ground quickly.
6. Don't be a dick. One of the things which will kill a game incredibly quickly is a GM that deliberately shits on their players for no good reason. Don't check to see if they fall over their feet when they walk or break their head open every time they climb a flight of stairs.
7. Be a dick. If a player is obviously trying to metagame or break the game in a boring way (
especially one that harms the experience of other players), don't be afraid to shut them down. There can sometimes be a fine line between this and creative play, so it's up to you to judge. I typically go by a rule of thumb that states that anything that makes the game a better experience for everyone involved is both good and allowable. If someone's just spamming random crap in a non-freeform game, don't hesitate to give them the boot, particularly if it's annoying other players.
8. Don't be a dick (again). A GM should be willing to work with the players and compromise when necessary to resolve reasonable objections to things which were unclear, situations where the player had zero influence or control, etc.
9. Account for player actions. Have the world change, possibly in subtle ways that they will never notice, because of the actions they take. This is in no small part to provide fun for you.
10. Don't panic over a schedule. If you don't feel like updating, it's better to wait a bit than put out a crappy update. Don't try to set yourself to something like an update every day, just do what you feel comfortable with. It's supposed to be fun for everyone, the GM included.