My biggest hurdle was sweep picking double octave arpeggios, couldnt get them for a long time. Then I spent a while listening to Jason Becker playing and imagined exactly what he was doing and all of a sudden the penny dropped - didnt have to practice them much after I 'got' the concept.

To get them tighter I used midi to help: open up a midi part in your sequencer then input an arpeggio and loop it. Start it off slow at say 80bpm and play along with your guitar - up/down/up/down until you have it nailed. Then increase the speed in increments untill you hit your maximum speed threshold. This trick really helped to keep timing when I was going to sweep something odd like a solo in triplets over a rhythm in 16ths or vice versa. Try it before you knock it!