It helps to stop yourself every once in a while and just notice what you're thinking. If it's negative, make the observation that you're on a negative track and then try to steer yourself to looking at the same topic in a different light.
Ex:
Thought: I'm totally going to fail my math test tomorrow. Then I'll fail math, then I'll flunk out of school, then I'll be totally worthless.
Stop yourself and think: I've worked as hard as I can and studied really hard. Maybe I won't fail after all.

This worked for me!