When I'm trying to get going really fast, I tend to lock my wrist and hand. (Think tremelo picking)

*Quote from Michael Angelo*

"The secret to being able to alternate pick fast is the ability to start a new string with an upstroke."

Just make sure your picking motion is the same fast or slow. A good exercise is to pick (no pun intended) one note on the fretboard and start alternate picking it, speed up and then slow down, speed up and then slow down. Make sure that your picking style doesn't change.

There is no secret on being fast. You just have to practice practice and practice some more. If being fast was easy, then everyone would be able to shred. Just don't give up.

I also suggest checking out Michael Angelo's Speed Kills DVD. It's very helpful.