1 - Start off slow. Trying to play this technique quickly right away will limit your future progress.
2 - Don't use your wrist. Let your arm do all the motion for you.
3 - Keep it simple. Start off with two string sweeps then move to 3, 4, 5, 6, or 7 string sweeps.
4 - Make sure to simplify you motions. In a standard sweeping motion, you will have one motion going up, and one going down. Do not try to individually pick different strings.