There are a couple of methods for mass removing snails. I agree with Styggian, don't go with a chemical method.
The first method is a snail trap. You stick a piece of lettuce in the tank overnight, and by morning you should have a decent herd of snails on it. You scoop out the lettuce and throw it away. Repeat until the tank is sufficiently snail-free. It won't kill all of them, but it will reduce the population. I should say this method never worked for me, but my snails don't seem to be in to vegetable matter that much.
The second method is to get some kind of snail-eating critter. This is only an option for a tank big enough to house them, of course. What size is your tank? In a small tank, I second the idea of an assasin snail.
The third method is a total tank overhaul. It's a pain, but it does reduce the population quite significantly. I did a couple of large rescapes lately and I found that the snails generally got stuck above the water line, where I could pick them out.
Of course, you can combine the methods. :)
There are also precautions you can take. I found that my snail population really grew when there was a bit of extra food in the tank. Now with many small fish, my betta girls and my cories getting it all, and the shrimp cleaning what they miss, there are very few snails left.