I am up in western Oregon. Here is what works for us... frogs, toads, chickens, snakes, birds and ducks, and ground beetles... no one animal seems to keep them down, but collectively can be managed. Also, these guys will eat slugs/snails different times of the year from my experience. Also, the size of the slug/snail makes a difference to what predator will consume it. Also, snails generally are usually out and about in warmer times, and is dormant in winters (which can be relative for us west-coasters). Frogs and Toads (year-round):
We setup a lot of toad houses all around the property, and we do not have a snail problem. In fact we have a 100-acre field behind us that is usually fallow but gets tilled (don't ask). In the low-spots of that field, frogs/toads congregate and I snatch some up and move them onto our property. We also get those really cheap solar landscape lights (.50-1$) and put those near the toad houses, so that other night-time insects as well as the snails/slugs are near them for even more food. Birds (year-round):
Most ground-feeding insect eating birds will eat small slugs/snails. Robins, Brewer's & Redwing Blackbirds, Starlings, Scrub & Stellar Jays. If you bring in Blackbirds and Jays, they tend to keep Starlings out (at least for us). Garter Snakes (late-spring/summer):
Stack-up little brush piles around the property or let grass go long if you can and you will not only get some slug/snail control, but they will take care of field mice, voles, moles and gopher babies as well. Unfortunately our property can is not "messy" enough for garter snakes (and my wife's strict no-snake policy) make it tough for us to keep any around for any length of time. Also we try to bring in raptors, and they tend to grab snakes from us.Chickens and Ducks (year-round):
We have chickens, and a friend has ducks and geese. All will eat snails/slugs for sure, but they are pretty destructive. Sometimes for us we put down cardboard and let a bunch gather up, then only the chickens around that small part. Also chicken-tractors work well for us.Ground Beetles: (late-spring/summer):
If you can have rotting logs around your garden or big sheets of bark (and mulch), you can really build up a good ground beetle population. I don't recall about snail eating, but ground beetles will eat small slugs and babies (so maybe baby snails as well).
We have been getting larger populations of Milky Garden Slug in our raised beds, so I will be building covers for our raised beds and putting our chickens to work in the beds for a period during fallow times. Our Black Slug & Banana Slug population has been really low for us even though we do have quite a bit of Doug-Fir on property. We almost have zero snails any more, but when we do it is a Brown Garden Snail type.
Depending on the size of garden/field plot you are dealing with, you might want to investigate the concept of Beetle Banks http://en.wikipedia.org/wiki/Beetle_bank
. I went to a talk where area-farmers (mono and poly croppers) were putting them in on the perimeters of field plots, also cutting there plots down in size to accommodate even more beetle banks.