When you have to offer criticism or disagree, recognize the other party's position first. Let them know that you empathize with them, that you agree in part with their stance, or that you assume that their motive is positive. By using this approach, you take the other person off the defensive, and allow them to hear your position. If you wait too long to speak up you risk becoming aggressive instead of assertive. In this case the other party is immediately put on the defensive and will be unable to hear the validity of your concerns.
Nonverbal messages have at least as much impact on the listener as words do.
Keep a record of the times you are able to engage in assertive behavior. This record will boost your confidence by allowing you to see the progress you have made. Most importantly, be patient with yourself. Changing lifelong patterns of behavior is difficult. You will make mistakes and fall back into old patterns, because you are human.