It is never easy to change any behavior. I suggest looking up Cognitive-Behavioral conditioning methods.
Aside from that, try wearing clothes that make you feel powerful or professional (suits, nice slacks) and standing up straight and tall at all times. Keeping an open posture with your shoulders back and arms un-crossed is an assertive body language that will make others think you are sure of what you are saying, even if you aren't.
While it is good to be assertive, remember that pissed off people still aren't going to listen. So, you need to listen to everything they say, and then assure them that you understand, and following that, convince them of your point of view. (Sometimes you'll need to confuse them to do this, but hey, you still 'win').
I definitely recommend starting with the body language that will make you feel assertive.