Speak slower, I've heard that a stutter is your brain moving faster than you're mouth. Try speaking slower and concentrating more on what you are saying if you think it's about to happen. My b/f gets a bit of a stutter if he's nervous and this seems to work for him.
I agree with Eve, you must really take the time to think, and know exactly what you want to say before you start speaking. I have heard of recording artists who stutter like crazy when speaking, but are smooth as silk when they sing because they have the words to the song memorized.
There are a lot of people that stutter really bad to the point that you can barely understand them, but then again they can sing what they're trying to say perfectly. Not that you would want to sing everything you say