I couldn't play the petrucci exercises. It sounds false (not in any scale), it sounds shit and isn't fun. You can get there if you have much discpline, but if you are like me you want to do it by playing, not practicing. (after all practicing is what you do in schooltime ;-))
Use the paul gilbert technique you can find here
http://www.intimateaudio.com/psycho_licks.gilbert.html
That's basically the way.
Don't learn his things; make your own licks/patterns. It's much more fun to be creative and see how it becomes. It doesn't really matter how it sounds slow, when you speed it-up it will sound cool.
I've made this one:
(put these in one beat)
|------------------------
|------------------------
|------------7-----------
|--7--9--11-----9--11----
|------------------------
|------------------------
I first couldn't make 80bpm on this. After some days (now) I can play it at 110bpm. Maybe this is not real shredding yet, but I feel improvement.
4 Sugesstions:
1) Use a distortion pedal. When you are bored of a sounds, change distortion.
2) Try it in different posistions.
3) Do not worry much about speed. Put the metronome on and play it. If you can play it well (and played it like 5 or 10 times) put the speed 1bpm higher.
4) If you are bored of it or so, do something else on guitar for 50 or 30 minutes. Then the exercise again. The key
is _keep_ doing it, do not change lick. Within some weeks you can do it real fast. Also maybe usefull is to use a clock or stopwatch. Put it at 10 minutes, keep continue playing.
5) Try few stringed first. Play it on one or two strings.
Also get the paul gilbert videos.