How to keep bags from floating in a sous vide supreme
Two things cause bags to fill with air during low temp cooking.  The expansion of air that already exists in a product (typical in vegetable cookery), and gas that results in bacterial growth (a problem with some cuts of meat).  For vegetable cookery weighting helps, but this is almost always an issue unless you compress a really hard vacuum with a commercial machine.  For most home cooks, the best solution is to use zip lock bags, monitor, and then release the air and reseal as it builds up.  For meat cookery, try pre-searing for 20 - 30 seconds per side...or dipping in boiling water to kill off any surface bacteria prior to bagging and cooking low temp.  Also the sous vide supreme comes with a rack to aid in keeping items submerged.  That can help too.