Due to the size of the tank it's pretty tough to give you a natural way to get rid of them.  (Some fish eat snails)  I'm not sure if you mean the long pointy snails or more of the round "typical" snails most people find.  If they are the pointy cone shaped ones they are probably what is probably a live bearing snail instead of the egg laying ones.  The egg laying ones have a somewhat clear jelly like egg.  If you remove the eggs and as many of the snails as you can the population usually comes under control.  To fix the problem with the live bearing snails you usually have to remove them from the tank.

There are some chemicals (Had a Snail, Rid Snail, etc) but these generally are somewhat serious chemicals and I do not suggest using these.  If you pull out as many as you can this tends to be the best overall because the snails do eat some of the waste in the tank and keep it somewhat cleaner then normal.  Additionally it is somewhat interesting to see the gravel "ripple" with the snails digging through it.  This tends to keep the debris in the gravel cleared out and protects the tank from having toxic buildup in the gravel.