Always consider player interaction before anything else.

RP's are like webs of character connections with content and posting responsibility flowing from one member to the next - a whole slew of mixed metaphors, you see. If your RP isn't set up well, activity will get clogged up and the whole thing will start to crumble.

I realize that was probably pretty confusing, but what I'm trying to say is, just try to visualize in your mind how the posts from each member (and from yourself) affect everyone else in the RP. You should always make sure each post you write contains something for others to react to, and ensure that the rest of your players do the same. Additionally, avoid scenarios where the characters have nothing to do, where a single player is writing interactions between two or more of their own characters, or where you just have too many characters in one place, so it's hard for the players to know who moves next, causing everyone to just stop out of nowhere because they all think they're waiting for someone else. Which brings me to my next point - if a player, for any reason, becomes inactive and starts holding up the activity flow, [i]act fast[/i], or else their absence could hold up a bunch of other people and cause a huge chunk of your RP, if not all of it, to crumble.

There are a lot of potentially bad scenarios to look out for, but basically what I'm saying is, always think of things in terms of activity flow. You're right that planning a novel and planning an RP are two very different things, because when planning a novel, you only have the story to worry about. But with an RP, you not only have the story to keep an eye on, but you also have activity flow, which so many GM's seem to overlook despite it being the blood that flows through every RP's veins. Always look at every decision you make not only from a story standpoint, but perhaps more importantly, from an activity standpoint. Always try to go for what will keep things flowing the best.