Sweep picking is the only technique that I had to get instruction on. It doesnt matter what you read, you will only grasp the trick behind the technique by actually seeing someone play it up close. Either get some lessons by someone competent or get an instructional dvd. The best sweep dvd's I have seen are "Sweeping All Night Long" by Theodore Ziras (there are some good clips on his website), "Speed Kills" by Michael Angelo or Frank Gambale's "Monster Licks and Speed Picking".
I found this the most difficult technique of all, it ook me about 6 months to get it. You might be better off leaving it for a week or two, then going back to it. Sometimes you can actually get quicker after taking a break.

Hope this helps.