Yngvai made a very important point.
RELAX! don't do it, when ya wanna...
oops sorry!

But seriously you need to stay relaxed.
when you tense up it becomes even more difficult to play fast and you tighten up. Everything becomes really choppy. I mean, you start to do things like lifting your fingers way too high off of the frets and slamming them hard down on the fingerboard. You lift your pick to far from the strings, that sort of thing. When you are tense, these things happen and the farther the distance from the pick to the string, or your finger to the fretboard, the slower you will be. (am I making sense or do I sound like Yoda?) :zombie:
Just pick some 3, 6, 12, or 24 note pattern. RELAX, play it slowy and build up the tempo with a metronome (I use MIDI drum beats). You just have to keep doing the same damn thing over and over again. It is very boring.
Like Yngvai, my strength is not alternate picking. In fact, without knowing it, my natural picking habit is to use an odd blend of alternate and economy. I won't even try to explain it. My biggest strength is definitely legato.