When I'm getting a track together I invariably come up with a riff or run that I just cannot play, so I write it out in midi then slow down the bpm to a comfortable pace and play along with it on a loop. Once you have it nailed just increase the bpm by smallish increments (eg 5 bpms) untill you hit the desired pace (eg start at 140 bpm then gradually build up to 170 bpm). You'll find that you can get there in a a very short period of time. This method has increased my skills dramatically, it works particularly well for straight forward alternate picking and sweep picking). One other thing I've notice beside an increase in speed is a huge improvement in timing and tightness/accuracy when it comes to multi track recordings.