Ok so i dont get it im just starting out on sweeping (yes ive checked the guides but nothing really address's this in depth). So then am i missing something as to mute the strings when going down? thanks =P.
You want to sweep downwards with your pick. If you rest the pick on the string you're playing next, before playing it, you'll naturally mute unwanted strings.

As for the fretting hand, although they're usually 1 note per string, you still want to play it like a scale. Therefore, take a look at Freepower's 'Muting unwanted noise' lessons. If you can't be arsed, basically you want to must unwanted strings with your index finger, and pull off each string as you play the next. Sorry i'm terrible at explaining, seriously look up Freepower's muting lesson.
Muting going down is alot easier than going up for me. I usually hold my pick parallel to the strings, and as I pass them, just brush the side of my palm against them, and then hold it there as long as necesary. That's just my technique though.
Muting strings in sweep picking is achieved by a work of both hands

the left hand avoids the note to keep ringing by quickly taking the fingers of the strings inmediatly after you play the note, that's probably going to make the string to ring open, to avoid that, the right hand mutes the string by just putting the palm slightly over the strings as you play

i'm not a huge sweep picking player, but i can get them cleanly and at a decent speed

try these videos, they explain (in a certain way, more visually than theorically)


http://mx.youtube.com/watch?v=Z2yNsCdbLd8 (yup... herman lee)

practice and one day you'll be like this