My guess is that you are speaking of your pick speed between strings instead of just tremelo (ing) on a single string. I used the following exercises to help improve my speed and accuracy.

1. String Skipping exercises. I used to economy pick these slowly and build up speed until I could them clean. Like Nick said, Paul Gilbert does a lot of this and his video could definately help.

2. Playing 3 or 4 note per string Scales and have them descend from the Low E string to the High E string and ascend back up to the starting position.

However, there is no real quick fix or secret to speed. These things will come over time and much practice will be necessary. Practicing correctly, however, will enhance and speed up the time at which you reach your plan.\

Peace.

-Kyle Hicks ( Kole )
-http://www.myspace.com/kolemusic