While performing a sweep, many times after moving my left hand fingers from one string to another, it will cause that string to vibrate and cause the open note to be heard. is it a matter of positioning my hand better for a more direct vertical attack to the frets, or perhaps with greater speed, my fingers will return to the string before much of its open note can be heard?
any suggestions or reference to a tutorial on how to prevent this?

thanks!

Im not the best sweeper, but I know your problem involves muting not attack, or speed.

For best tutorials, check the forums, and Pavel's 3 string, and 5 string sweep lessons. Pavels, is very good not sure about the newer stuff though.

Good answer Ryan - muting is the key! Ivan has done a great tutorial on this topic.

The idea is to mute both with your right hand palm and left hand fingers. If you attempt the sweeps really slowly - you should be able to figure out when to mute the unplayed strings with which parts.