Most people will say play it slow, and slowly build up speed. This is not wrong, but it may not work for you. It didnt for me.
Do not take what i am saying as the absolute truth, but different people learn differently. The way I approched playing fast is more of the Shawn Lane/ Chris Impellitari way. I took simple patterns, for example:
E--------------------------------------------------------------------
B--------------------------------------------------------------------
G-----------------12--13--15-----------------------------------------
D---12--13--15-------------------------------------------------------
A--------------------------------------------------------------------
E--------------------------------------------------------------------
and would play them as fast as i could, forward and backwards. Then i would alternate by playing the indivdual 3ps pattern backwards ascending, I.E. 15.....13.......12 on the D string 15.......13.........12 on the G string. etc. I would try to do it as many ways as i could break up the patterns, groups of 3's, groups of 4's, 5's etc.
At first it was utter chaos and slop, but instead of slowing down i slowly cleaned it up. For some people like me, in order to play fast you have to know what its like to play fast. You can't know it until you do it.
I tried the other way, and i just got no results, i banged my head against a metronome for almost 9 months with no real results. Until i saw this video by Shawn Lane and it made sense to me, after that i was playing things i never thought i could play.
http://www.youtube.com/watch?v=DhkbSBxPYcUBoth methods can and do work, the question is which is right for you. The only way is to try each of them
Daniel