Everyone has their own idea about roleplay and their own style, but in order to play well there are general rules that should be taken into consideration.

Have a clear idea before you start. Think about what your character looks like, their past, attitude and beliefs. Write/type down anything you need and expand on it whenever possible. Remember that others will find it easier to roleplay with you if they know what your character looks like.

Try to be consistant. It's very unlikely that a vampire will suddenly follow Light after being a joyful killer and follower of Darkness, these things do happen but not overnight. If it is important for your character to have a change of heart then roleplay into it slowly, take your time to build up to the eventual change.

Present yourself well. Chat speak in IC channels doesn't reflect well on your ability to roleplay, check your spelling if need be and remember that IC can and will be very different from OOC. Speak in IC channels as your character would speak as a real person, try to make your character seem like a real person. (Unless you just so happen to be a Troll that is :P)

Try to follow the theme of the world. Simple things like children out of wedlock and adultery can be taken rather more seriously in a medieval world than in current times, popular swear words are not likely to be in keeping and some are more insulting then than now. Plus swearing would be considered very crude and common in a medieval theme. ;)

Roleplay is about quality not quantity. Maybe you write rpnotes once in a blue moon, but maybe they are very well written and insightful too. Lots of badly written or badly spoken roleplay can be damaging, take more care and time over what you do and the difference will speak for itself.  (Unless you're a Troll, yet again :P)

Don't be pushed around. If you feel uncomfortable about anything being roleplayed with you then put your foot down. Make it clear OOC and/or IC that you aren't happy with the situation, never allowed yourself to be forced into anything you don't like.

Basic things to remember about your character description:

Think of colours, eye, hair, skin etc.

The clothes most often worn by your character should be mentioned.

Try to avoid forcing actions or reactions on others, let them form their own opinions.

Remember what kind of world you're playing in and keep to the theme as well as the content.

Don't worry about changing it when need be, you can't expect your character to wear the same clothes all the time. :P

Basic things to remember about your character history:

Be original, give yourself an interesting twist for people to learn about.

Don't give it all away at once, the mysterious stranger is fun to play with. ;)

Think about how it will affect your character, maybe you fear a certain race because of something that happened when you were a child.

As always roleplay can and will be a serious thing, but it is just part of a game and you should never be upset by anything that happens.

The last, and most important, rule you should remember is:

Never mix OOC and IC.