I use a nymph or a scud sometimes under a VOSI (link below) float usually when straight lining is not working (on the drop or the retrieve) and fish are suspended, picky, and seem to want a nymph or scud suspended at a certain depth. If the wind is blowing (usually the case) then I rarely even have to move the float (has enough motion on its own) but if there is no wind and its pretty still, then I may give the line a twitch once in a while to give it a bit of movement. I have also had pretty good luck at times with a Cypert's Mylar Minnow under the VOSI float.

[url=http://laflyfish.com/flies/vosi.php:609ce]Vertically Oriented Strike Indicator (VOSI)[/url:609ce]