1) Build the Action: Try to put your energy into building on the drama and action of the game whenever possible by speaking with energy and enthusiasm when the action calls for it. If something interests you then show the GM by asking questions about it or in other ways.
2) Prepare: If you are the type of person who has a hard time getting into character then you should prepare a few lines of dialogue or other ideas that will help to facilitate the action at critical moments. Catch phrases, if done tastefully and sparingly are usually a good start just don't steal them from movies if possible. Try working on a simple accent or a few phrases that your character might say either taken from their heritage or other in game dialogue.
3) Stay in Character: Players should try to remain in character at all times. Even if you need to do something "out of character" try to keep it short and sweet and get right back to the action. Try to avoid words like I roll to hit or I dodge them or etc. Again less is more.
4) Leadership: You should either show or follow leadership to help keep your party on track whenever possible. A strong leader can be a good way to get a party into character so if you are the strongest roleplayer then help the group through your example.
5) Leave Room for the GM: Remember that good roleplay is important but there is a fine line between enough and too much roleplay. Be sure to allow the GM to control the flow of the game and use roleplay to fill the gaps in between.
6) Timing: Save most of the dramatic dialogue for a big battle or a worthy adversary.
7) Interaction: Watch other players (and the GM) and see if they are having fun and if not try to use roleplay to get them involved and increase the fun.
8) Beware of Overacting: Overacting or mechanical acting can be more of a distraction that an help. Try to imagine how you would feel in a similar situation and channel that emotion into your character.
9) Relate to Your Character: We don't recommend that you choose a character that is too different from your personality (especially at first) so that you can use personal feelings to help to create dialogue.
10) Consult the GM: When in doubt consult with the GM about how you might typically feel about people, surroundings, and situations.
11) Get to Know Your Character: If you are having trouble try to get to know as much as you can about the class you are portraying and the skills you have chosen. Some characters even write a diary of a game in the voice of their character.
12) Body Language: Body language can add an element to your character that cannot be filled by other means. Gestures and expressions can really bring a character to life.
13) Stay Aware: Keep alert while playing to look for opportunities to roleplay.
14) Speak Clearly: Unless there is an "in-game" reason to do so (such as you are whispering or etc.) then try to speak with a clear voice. Tone should range from average to loud depending on the circumstances.
15) Remember to always have fun: This one is pretty self explanatory. Just remember that the ultimate goal of role-playing is for everyone to enjoy themselves so Don't stress over the small details just relax and have a good time.