Better lighting setup to light the screen evenly, repositioning your lights to better locations, more distance between you and the screen (most common cause is because you're casting a shadow on the screen, which you can't allow to happen), and tuning your key value, similarity, blend and spill reductions very carefully.
Also, if you're getting sparkles from any creases in the muslin, do yourself a HUGE favor. Buy a cheap $30 garment steamer. You can't iron muslin without risking damaging it, but steaming it works absolute WONDERS.
http://www.ferretbomb.com/steamer/before.jpg
http://www.ferretbomb.com/steamer/after.jpg
The top of the 'before' is smooth because I did one side, then pulled it back so the whole screen was hanging off the ground; so the top two feet or so had already been steamed. SHARP difference. Plus, you can use it on your clothes to get that fresh-pressed look. :D
