Everyone has a different method of dealing with stains. Here are some of the most popular:
Sunning - by far the easiest, most effective, and most ecofriendly method. Simply lay clean, wet/damp diapers out to dry on your deck or use a clothesline. The sun can work wonders in 2 or 3 hours.
Oxyclean - considered safe to use by most, oxyclean boosts your detergent and helps remove stains in the wash.
Baking soda - can help brighten dingy prefolds, but doesn't have the stain removal power of the sun.
What about bleach?
Never use bleach on natural fabrics if at all possible. In fact, you shouldn't use it all if you can. It's a very harsh chemical to place on something that is going against baby's bum. Some companies recommend bleaching microfiber inserts once a month (they really hold onto stink after a while). If you do this, rinse, rinse, rinse!