Instructions
Create a character with a "realistic" name. While crazy fantasy names are fine, especially if the player did a little research to make it match their race well, people often frown on crazy names. Players will roleplay with a gnome named Genzil more easily than one called HaCksJoO.
Learn as many emotes as you can. Emotive action and discussion are really big among role players. Even when you're out in the world, waves and bows are often more common than any speaking. There are a lot of emotes built into World of Warcraft and the "/em" command allows you to make your own.
Role play only when appropriate. If you are dealing with somebody that obviously isn't role playing, don't try to force him into it. Simply conduct your business and be done.
Build a good reputation. Avoid role playing that would create a bad impression among your teammates. You may have the persona of a cowardly rogue that flees from danger, but doing that in the middle of an instance will get you blacklisted pretty quickly.