1. Read (as stated above). The more you read, the better you'll write, because you'll increase not only your vocabulary and your grasp of various subtleties of grammar, language, and syntax, but your overall knowledge of the world and its people. So, unless you only ever intend to RP yourself, or a mirror image of yourself, you'd better learn as much as you can about *other* people, cultures, ideas, time periods, etc.

2. Practice writing! Don't just write when you RP with your friends. Write stories. Make up new characters. Don't be afraid to suck at it, at first. The only way to learn is by trying, failing, and learning from your failures. Write a story for a new character, leave it for a few days, then read it again. Sometimes, just a bit of time in between writing and reviewing gives us a fresh perspective for things we could improve, or things we might have missed. 

3. Be teachable. The most unbearable RPP's are the ones who think they know everything and are perfect masters of their craft, worthy of peering down their noses at others. Even if you dislike another person's style or material, you won't get anywhere by being a jerk or a snob. If you don't want to RP with someone, then don't. If they seem open to critique, offer it politely, then allow them to accept it or reject it, without turning into an ass if they don't embrace your opinion. By the same token, be willing to hear "I think you could have done this better" without snapping in defense, or you'll only wind up limiting yourself; not only in who'll RP with you, but in what you can learn and how you can grow.

4. Remember that there is no story, no character, and no RP that is more important than the people behind the characters. I can't stress this enough. If you value a fictional character more than the flesh-and-blood human being behind the other screen who is providing that character for you to enjoy, you need to re-examine your priorities. 

5. QUALITY OVER QUANTITY. I have to respectfully disagree that being overly descriptive trumps a story with some depth and substance to it. You don't have to make every post a novel. If you can convey your idea in ten words rather than a hundred, sometimes that is *all you need to do*. I don't need to hear about your "plump tiers curling slightly upwards at the corners" every time your character smirks. I really don't. I know what a smirk is. Describe it once, maybe, then move on. No amount of flowery descriptives will make your story interesting. If it lacks substance, all you have is a very boring and uninspiring list of adjectives.