1. The PickSweeps are best done using a small to medium sized pick that doesn't easily bend. The pick shouldn't be paralell to the strings during the motions.
2. The ToneSweeping sounds best when using either a reasonable amount of distortion, or a clean sound with some compression added. Using too much distortion will render the arpeggios muddy. In general you should be going for a liquid distortion sound, if you know what I mean. John Petrucci is one good example of that kind of distortion that lets leads sound very clear despite the overdrive that's been dialed in.
3. The MotionsI've noticed that one of the main reasons for mistakes or sloppy sweeps is the player failing to attack each single string while performing a downward/upward motion. Make sure your pick is "falling" onto each single string as you sweep.
Another source of trouble concerns muting the notes/strings you don't need. I strictly use the right hand for this.
4. The ShapesCertain arpeggio shapes are less likely to sound clean, while others will make everything easier. In general you could profit a lot from avoiding a shape wherein two notes share one fret.
5. The Aesthetic AspectAlthough sweep-picking lends itself for some impressive feats of technique and finger dexterity, it's commonly used to play arpeggios really fast. So fast that you can no longer make out what kind of notes are in it. I don't know how fast I can play arpeggios using sweeps, and to be honest I don't think it's important, as I only use the technique when I feel like adding some harmonic texture to a solo.
6. The EtudeThe most fun, musical way to practice a new technique, or a technique that's giving you trouble, is coming up with a short piece which solely revolves around that technique. That way you'll be both practicing and making music.
I do this all the time, and I can't recommend it enough. Here's a short piece I wrote to showcase my own approach to sweep-picking. I hope you like it.