For success against slugs and snails, one word – P r o a c t i v e ! If the first food they get upon waking from their winter slumber is slug and snail bait then they will never get to your plants or be able to reproduce.
Photo credit: jacki-dee via Visual hunt / CC BY-NC
I had big issues in my garden and started off with a duel approach – I used both an organic bait and a foliar spray on the plants in the beginning of June. The first year, being diligent with reapplications, knocked the pesky critters back considerably. Following years were less work as long as early June applications were made.