First, after tying in the tinsel tag, run the thread (I always use white thread when I'm using flourescent floss) to the forward-most point of what will become the butt and tie in the floss like this:
I use Gordon Griffiths floss for my flies. Its hard to come by, so I'm always on the lookout for it. I think its depth of color is unparalled and that alone would be reason to use it, but its also easily split into two halves. I think I get a smoother, sleeker butt or body by just using a half. If you like ridges in your butt or body (keep your mind on the fly), then don't split the floss. I do know some tyers that like that look better.
Well...wind the floss back to the tag and forward again to the tie-in point and tie off with a couple thread wraps:
Now bring that little bit of floss up over that tag, make a thread wrap, give the floss another little snugging tug, and tie it in:
Now your butt won't drag!
I can't keep myself from adding a little extra note to this post. On one of the internet forums I frequent, some fellows were extolling the virtues of cheap flies recently, as low as 50 cents each on the internet's most popular auction site. They even mentioned a couple of that site's online fly shops that had "great, cheap flies". Well, speaking of dragging butts, here's what those guys thought were a good deal (photos from the auction site...THESE ARE NOT MY FLIES!!!)
Some folks are easily pleased, I guess. I sell my hairwing salmon flies for four dollars each. At least they don't start out looking like they've been fished for a hundred years! Never was this more true, in my opinion: you get what you pay for, fly-wise. And finally, as Jerry Doak says on his website (http://www.doak.com/), "...the fly is the only part of your tackle that a salmon sees, so isn't it the last place you should cut corners?"