well, I am getting better and better at breaking my own bad habits. The first step is to learn how to recognize as soon as you do "it" and correct the behavior.
2 suggestions: have your good friend(s) who you are around a lot subtly and nicely point it out to you every time you do it for a few days. By then, you might start to notice when it happens. But make sure it's a good friend that won't embarass you or kill you self-esteem!
And maybe tape record your voice so you know exactly what it sounds like.
Oh, and, if you want to spend $$ (hey, lots of college students have it -- not me
) You could hire a speech therapist. Maybe some of the students in the speech therapy program, if you have one at your school, would be willing to try to help you for free.