1. Set your limits for any job when you write (i.e. before you submit) your contract. These should be time and work quality as well as money. Sometimes the act of writing this down is enough to remind you at the outset that you can't afford to cut your prices 45%. . .

2. Here's a tip from the billing and pricing meeting I run: think--and train yourself to talk--about the relationship between work and its fees not as "this is what I charge" but "this is what it costs." If the client then says "I'll pay you X" it's easier to reply "well, for X I can do Y but not Z as well."

3, Practice. And remember, as VG indicates, that it's not in your best long term interests to accept every job that you're offered.