well I'm not the fastest but I can give you few pointers. Practice your scales a lot and try to keep your fingertips less than half an inch away from the strings when you raise them up so you need less time to bring them back down. Getting used to this is actually surprisingly hard. I'm working on it now myself. Also do a lot of those excercises that go up and down the neck for warmup. Here is a link for a Yngwie Malmsteen website that has a lot of those excercises. Go through the ones titled need for speed.

http://www.yngwie.com/Articles/

The important thing is to be able to synchronize your hands so that you pluck the string at the same time moment when your fretting finger comes down. If you don't do this your notes will sound sloppy. Only start increasing speed if you are comfortable playing whatever you are playing at a slower speed. Don't just try to go as fast as you can. That's how you get really sloppy sounding stuff.

hope that helps

good luck