Sweeping is one of my favourite techiniques and I try to slip it into every lead that I can. It took a long time to come to grips with it and I have to admit, I got nowhere from any internet lessons. I learnt it through trial and error and eventually got to the stage where I can sweep just about any arpeggio (except for the odd Becker patterns!).
The best way to learn them is to actually listen to players that play them perfectly eg Vinnie Moore, Jason Becker, Joe Stump, Michael Angelo and Theodore Ziras. The more you listen to them, the more you can imagine how they are actually played, this really helps the "penny to drop" as this technique is not about having fast fingers, its just about linking your mnd and fingers.
Here are some links to sweeping masters:
www.angelo.com (download the MP3 for "No Boundaries" - crystal clear technique)
www.theodoreziras.com (check out the downloadable videos - this guy is an excellent technician)
http://www.vinniemoore.com/ (download "Defying Gravity" and "Awaken The Madman" - this guy is the only player that plays sweeps patterns that I would consider impossible - also one of the underated guitarists in the world, incredible technique that manages to still sound musical).
Just keep ripping away at the sweeps, try playing them too fast, sometimes you stumble over part of the puzzle by getting the right hand at the right speed. Once the technique suits you, try slowing the right hand down untill the left hand can match the rhythm. Try three strings first, once you can get them it's not much differnt going to 2 octaves over all 6 strings.
Best of luck