It never hurts to be skilled when it comes to standing up for yourself, whether it's to your boss, a nasty editor, a crabby source, your girlfriend, whatever. See Worldwit's article on "How to Speak Up without Causing a Blow-up."

Sometimes you gotta psych yourself up:

Reverse your thinking. Most of us decide whether or not to speak up by considering the risks of doing so. Those who are best at crucial conversations don't think first about the risks of speaking up. They think first about the risks of not speaking up. They realize if they don't share their unique views, they will have to live with the poor decisions that will be made as a result of holding back their informed opinions.

More here.