From little experience - one snail equals HUNDREDS!
When you buy new plants at the LFS, be sure to really clean them well in the sink before inserting them into your tank.
The egg sack for the snails is a gelantanous (sp?) clear, sticky sack with white dots. They will lay them on the walls of your tank, on the plants or even on any of the decorations you have.
I would scoop them out 20 at a time and the next day it would look like I hadn't done ANYTHING.
They add greatly to your bioload too!
I finally caved and bought a clown loach. That one fish will mean I can't buy other fish because of how big it will grow - but snails are the bunnies of the water world! They multiply faster than anything I've ever seen!
Good Luck!