There are a number of fish that prey on snails, but whether they suit your needs will depend on the available bio-space in your tank and what stock you already have.
There are chemical treatments, but I would never recommend them in the least. Putting toxins in your tank is just a bad idea all around.
While you're working out longer-term solutions (possible snail eaters, correcting feeding/cleaning habits etc), handpicking snails can do a remarkable good job at keeping their numbers down if you simply make it a habit to pull visible snails. I've gotten pretty good at fishing 'em out with my planting tongs. I also keep an eye out for snail eggs and net them out whenever I see 'em.
If you've the stomache for it it and have omniverous fish, squishing them against the glass kills the snail and gives your fish a treat.