It depends on how bad an infestation you have.
First thing to do is to cut down on feeding your fish. The main reason a pest-snail baby-boom occurs is because there's is plenty of food for them to feed on. Cut down on your feeding, and they won't reproduce as much. Remember that your fish are perfectly capable of going a week and more without food (they would do so in the wild regularly). I feed mine once a day, sometimes giving them a break of a day.
After that, pop in 2 or 3 assassins, they won't eliminate the problem overnight, but give it 3 or 4 months, and you will see a difference. And a small pile of empty snail shells.