As I always say, if you want to fly you gotta crawl. no joke, I've discovered that they best way to get fast is to go slow when you practice, know where you are going (so a god knowledge of scales, have a sense of the melody you want, etc.) and a metronome isn't a bad idea (but I like drum machines or backing music better). And, yes, relaxation is key too. Far too many people treat shredding like a sport or hard work. I take a "Tao of Shred" approach (only half kidding ;) ) that centers on being totally at ease when I play and just letting things flow.

Also, you might consider looking into sweep picking. I've used a modified/alternate form of sweeping to develop speed.

http://ShredLikeHell.com