Know the Status Quo
Embrace the status quo. Know what would happen if your character did nothing, and you'll have figured out 99% of your character motivation.
It's not all about you
You can't have a plot without other people, so put the focus on them.
Don't load your character up with too many skills. Let other characters do what they were designed to do and work
with them.
Plot the situation, not the scenario
I'm hesitant to even use the word plot. It literally means the flow of one event to another. Roleplay doesn't work well that way, because we can't plan what other people are going to do with their characters.
A better way of working is to throw your character into situations (or create them) without trying to plan
how they'll get from A to Z. More on this in another thread.
Accept possibilities
Accept that you won't have figured out every possible solution to problems characters could come up against in your plot. When, inevitably, other player characters come up with a solution you haven't thought of, accept it and roll with it.
Do not, I repeat, DO NOT, negate these type of solutions reflexively. If you absolutely have to do it, have a good reason; something more than "I didn't plan it that way".
Know how to meta
Meta-play is kind of the opposite of metagaming (if you want to know why I chose that word, just look up the origins of meta). It's the stuff your character would know just from observing the situation. If you've ever been in a thread where you described it as daytime and the next poster said it was night, you've experienced bad meta-play.
Good meta-play helps you judge what other players want from a thread, what other characters motivations are and helps you push the thread in the right direction to help them. It doesn't mean giving up your own desires, just that you get to take other people into account as well.
It also helps you develop really good brainstorming skills. It's also really important for freestyle plotting for reasons you'll see in part two of this series.