DO: Always obey the directions of the GM (Game Master)
While there are general role playing rules that are typically universal, keep in mind when you enter a game realm that you are in the Game Master's territory. Whatever rules, limitations, or directives the GM chooses to put in place should be followed - regardless of traditional RP formats or guidelines.
DO: Play to Rank
Regardless of how powerful you intend on making your character, if you are playing in a world that has a ranking system you must keep your current rank in mind during game play. In other words, trying to kill someone in a battle when you are a level 7 character by using a skill that you do not acquire until you are a level 10 is a big no-no.
DO: Separate Your Speech From Your Actions Using Proper Cues:
When you are role playing it is important to remember that you are writing both dialog, actions, and sometimes scenery or background. To keep these different elements straight, you need to learn the various cues which set apart speech from actions, etc.
One of the most common ways of doing this is to use the asterisk (*) as an action marker to set apart actions. To further clarify, you can put your dialog in quotations, although sometimes you may find that quotations are not used for dialog and only the actions are set apart. The following is a simple example of how this is done:
*a tall woman appears from the shadows with long, deep red hair dressed in a deep dark purple velvet dress with matching choker and a modern black trench coat in a tailored military style. Smiling slightly as she nods her head in a subtle bow*
“Dark Greetings to you, may I welcome you to the realm."
Other variations also include using a double colon (::) as the action marker before and after the action. Also, some people may or may not use quotations for speech so it is often the case that text without any identifying markers is assumed to be spoken by default. The screenshot below taken from the Bloodletting RPG shows what this would look like in action:

click to enlarge
DO: Use Proper Grammar and Correct Spelling.
Role Playing is an art form the relies heavily on other players being able to understand you. You should not have to stop and try to guess at what the other person is trying to say in the middle of a battle. This just causes confusion and may end up with very few people willing to RP with you.