1. Write your actions as actions, and your words as words. A good RP should be able to be read as a novel. Never interject actions in the middle of character speech.
2. *ALWAYS* leave other characters and the RP Master an "out" when you perform any action. Don't be afraid to incline the story in a certain direction, but don't force it in that direction either. Don't make the "out" incredibly difficult to work around to. People don't like being backed into corners and being forced to act a certain way. Everyone else in the RP has their own creative ideas too, try to work around them to create a masterpiece story!
3. Incredibly powerful characters are an irritation and a bore to everyone else. If your character was allowed near-godly power, don't use it to solve every situation, until you check with the game master. A good RP could have a single event unfold over several pages. You shouldn't be able to solve something all by yourself, or go from dire straits to instant victory. If you give yourself a powerful ability, add a "negative" too. Maybe you can't use it for long, or it has adverse affects on your character, it only works in certain situations, you leave yourself vulnerable while performing, etc.
4. If you have not developed your character before, spend at least a half hour thinking up physical attributes, personality characteristics, a brief history, etc. But don't reveal your entire character in the character sheet - it's always fun for others to find out something interesting about your character as the RP unfolds.
5. Keep things plausible, and within the confines of the RP setting. Keep your characters consistent to their character sheet, but don't be afraid to allow them to adapt and change - in fact, in a good RP, your character will change as he or she would in any best-selling novel. There should not be ancient underground temples in the Midwest of the United States, for instance. If your character is shy, he shouldn't be running around greeting every random stranger he meets.
6. Check your posts for grammar and spelling mistakes. This may be the internet, but people appreciate it when at least this small amount of time is given to your post.
7. If you can't contribute anything to the RP at the time, then don't. We want to hear from your character, but meaningless posts that don't bring the story along further, don't tell us more about your character, or don't open up new options and directions for the RP are irritating and just fill up the RP with spam. I usually spend at least 10 or 15 minutes on a single post, checking to make sure it fits within the guidelines of a good RP. I've started to write out posts, realized I was going nowhere with them, and then didn't post anything until later.
8. Don't blast miles ahead of the rest of the RP'ers just because some of you happen to be online at the same time. You are missing good opportunities to interact with other characters during parts of your "tangent." I have definitely been guilty of this in the past
. I know RP'ing is a lot of fun, but it is frustrating for others to have missed several pages of participation.
9. Be as respectful to other characters as you would your own mother. There are real people behind these characters, so try to interact with them with that idea in mind. Don't force them into situations without an exit. Find a clever way to present a situation, and then stop your post before you end up dragging others along with you. Basically, if you couldn't do it to someone in real life, then there is a good chance you should not be doing it to people in the RP. There is a fine balance between doing nothing and doing too much, and you've got to find it to make an RP fun for everyone. Some people will catch on to this quickly, some people will take some time to see it, but it is there!
10. Have fun! Don't be afraid to step outside the box, but if your idea is wildly outlandish you may want to check in the OOC thread to see if it is ok with the other people in the RP. Playing in an RP is a chance to become another person, maybe an ideal version of you, or maybe just a fun creation. You've got complete control of your addition to the RP, try to make it as enriching for you as for others.