DE is sold in garden centers. Dust or sprinkle it in areas you want protected. Be careful that you don't inhale the dust. You'll need to reapply after rains. Crushed egg shells also help.
Some people like to use saucers of beer to attract the snails.
Dispose of the pests after they've fallen in. I'm always hesitant to use product names in this column because a number of products on the market can help solve various gardening problems. But there is a lot of talk about Texas Pure Industries' red cedar products, and sometimes talk warrants a reference to a particular brand. See today's Lazy Gardener for other comments on this product. Persistence is the answer in trying to control snails.