This is tedious, but it will save you pain potentially. When practicing speed, get your metronome and put it on 40bpm, now play whatever scales you're working on using whole notes (takes forever), do all your positions blah blah blah, bump it up 3-8 BPM, do it again.

A lot of kids who shred now a day are just utilizing a muscle spasm, and thats kind of cheating. (and super lame)
If you want to play fast, you should be able to play whatever you're doing at whatever speed, which you can't really do if you're doing the whole muscle spasm thing.

Starting off slow also helps with phrasing, and accents as well as warming up.

Remember to stretch before you play, but not hype rextend, and always cool down when you are done.