Here's a great way to increase your speed, timing and clarity on arpeggio sweeps. Open your midi sequencer and program in an arpeggio and loop it so it plays forever. First try single octaves, then double and finally triple (tough!). Start at a low manageable bpm, then gradually build it up. Anytime I have to play a long sequence of arps I practice them this way first - especially if they are new patterns or have difficult fingering. You will find that you 'get' the rhythm quite easily this way and once you actually understand the tempo it becomes easier to play. I found that playing sweeping was more of a mind game than anything else, like Akira mentioned. After that it's a matter of adding the techniques the other lads above pointed out.