Here is some more advic, from an acwuaintance of mine on another rp site:
Well, it really depends on the person. Good RP'ing is partly about being a good writer, which is a skill you have to learn, and partly about knowing how to write a good story. That second part is about experience, but also about choices. You have to maintain a balance between various personal aspects that make for a good story-maker. Be kind, be inquisitive, and strive to become better. Try not to win, but try to show something interesting. You are a god in your own world, and to see a god perform miracles is not interesting. If your character can kill 15 men in a minute, that character is not interesting. You can make a character as powerful as you want. It is far more interesting to make them weak. That can be weak physically, but also weak mentally. We all have mental or emotional weaknesses, and when making a character, you have to take that into account.
When writing, write like a real human. Every human is complex, and to undercut that by making cardboard cutouts is a show of arrogance. Other people feel just as much as you, and you have to know that to write like a real human being. Have faults, make mistakes, have memories, have thoughts. No-one is evil for the sake of evil. Everyone is good in their own eyes. That has to resound from a character.
Basically it depends upon the rp type, as well as whether it is a 1x1 or group rp.