I dip them into a 2:1 hydrogen peroxide to water solution and then I have a 2 1/2 gallon tank with plain water I put them into for a bit to let the plants relax so it's easier to spot nasties before I put them in my tanks.
There are several recommended treatments including dips in dilute hydrogen peroxide (H2O2), potassium permanganate, and mild (5%) bleach. All can have some positive effects, but some damage sensitive plants. I have not found any to be 100% effective; especially dealing with snail eggs.
Today I use the quarantine method. I purchase a plant and it goes into a quarantine tank or tray with moderate light above it and water mildly fertilized with a balanced general purpose fertilizer like Seachem Flourish Comprehensive. I also add 2 -3 drops of Seachem Cupramine per gallon of water in the aquarium or tray. The copper in the Cupramine kills the snails, but not the unhatched snail eggs. I continue the treatment for about 4 weeks depending on water temperature to insure all snail eggs have had sufficient time to hatch. If I am treating the water in a tray without filtration I change the water 1 - 2 times a week as needed to avoid surface scum and stale water. Rinse plants thoroughly after treatment to remove copper, avoid using plants treated this way in aquariums containing shrimp or other invertebrates. All of my tanks are Snail-Free.
Better yet, purchase plants from forum members that are snail-free to begin with!
30 Gallon Snail-Free
Roy 75 Gallon, 2X55W AH Supply CF 8800K, 1XFluval F&P 2.0 48"/59W, 2X Marineland 350 Magnum filters; 45 Gallon Tall, 96Watt AH Supply CF 6700K, 1X Marineland 350 Magnum filter; 30 Gallon Long; Fluval F&P 2.0 36"/46W, Marineland C-220 filter; 20 Gallon, 1X26W AH Supply LED kit, Marineland C-160 filter; all with Press. CO2 and (Calcined) Montmorillonite Clay Greater Seattle Aquarium Society (GSAS)
I use an alum dip, 1 or 2 Tblsp per gallon, soak for 24-48 hours depending on concentration (weaker solution soak longer). I only use a weaker solution for really delicate plants, and I've left plants in for longer than recommended with no problems. It's a very gentle dip -- I used bleach once and it was hard on my plants. Also didn't like having to stand over it with a timer! You can get alum in the spice aisle of the grocery store, McCormick sells it.
I use the jungle clear water for snails. I use about a teaspoon per gallon in whatever container you are dipping them in. It's pretty mild(much less concentrated than the crystal form). I've left xmas moss to soak in it for half a day without it doing any noticeable damage. As others have said, it takes longer than other dips, but it's worth not defoliating your plants with H2O2, bleach, or excel.