Wow, such a short question with such a long answer. You may want to consult the lessons section here on UG. If I were to give you a short answer, I would say: keep the pick stroke moving evenly, mute strings on the downstroke as you sweep, and make sure each note gets an equal amount of time. No matter how fast you sweep, if you spend too much time (even if it's a thousandth of a second) on a note, it will sound sloppy.
Simple answer: Practice.
Quote by Geldin
Junior's usually at least a little terse, but he knows his stuff. I've always read his posts in a grouchy grandfather voice, a grouchy grandfather with a huge stiffy for alternate picking.
Besides that, he's right this time. As usual.