I'm not sure what kind of snails you have, but if they're pond snails, only one method that has worked for me so far... Make them gather in one "feeding area".
THE TRAP- I wrap #7 knitting mesh (yea yea, I know I use mesh for everything) into a cone shape & stapled the bottom closed. Then, I dropped a small rock down in there to keep the cone upright & I hung the cone so that the opening at the top is right at the waterline. That way any tiny snails can crawl in & out thru the holes, but the big ones that climb in the top get trapped for easy harvesting with a small plastic spoon. The unintended benefit of the mesh was that I end up "scraping" more snails off the side than I manage to get from the bottom. I leave the cone in there 24/7 & only feed in that one spot, so there's hundreds of snails in that area. I sometimes just drag the spoon up the side of the tank real slow & pull out a big mass of them. The dish/lid method got a few, but nowhere near as many as my cone.
THE BAIT- The best bait I've found so far is those dried sheets of algae they sell at the LFS & algae wafers, but the algae wafers smell horendous when you pull the cone out, plus they make a mess. I only had marginal success with lettuce, but cucumber & frozen peas worked pretty well. Since my snails are strictly feeders, I also drop random pellets & plant trimmings down in there.
For my Ramshorns, I use the same concept (gathering place/feeding spot), but they're pretty big, so I dont need the cone.
GOOD LUCK!!!!