From Peta's website:
Those of us who garden know that sometimes slugs can disrupt our efforts. There are many products that one can buy and methods that we could use, but most of those are cruel, may put you or the animals in your home at risk, and involve killing the slugs.
That is why I wanted to share a few easy ways to prevent slugs from ever entering and destroying your garden without inflicting any suffering on the slugs themselves. Prevention is a pretty simple (and inexpensive) path to keeping your garden healthy and slug-free.
1. Be sure your plants are trimmed back so that they receive plenty of air circulation. Crowded plants or dropped leaves that are allowed to stay on the ground are a haven for slugs. Slugs thrive in moisture and dark places, so the more air and sunlight a garden receives, the less likely it is that a slug problem will exist.
2. Planting marigolds in the garden is a great deterrent. Slugs are repelled by the fragrance (or odor) of marigolds.
3. Placing a copper strip an inch below the soil line and an inch or more above will create an electrical charge that actually deters snails and slugs. Inspect it daily for any fallen leaves—slugs can use them as a bridge over the strip. If you’re unable to locate copper strip, remove copper wire from an old appliance or lamp cord, wrap the wires around a pen or pencil to create a coil, making sure the threads overlap to form a barrier. Place the coil around the bottom of the main stem of the plant—leaving plenty of room for it to grow.
4. Install granite rock around the garden. The sharp jagged pieces make it difficult for slugs to cross because of their soft bodies. It will also add beauty to your garden.
5. Roof shingles can be placed around the garden—they are jagged and also act as a slug deterrent.
6. Sprinkle coffee grinds around the plants that are being affected. Caffeine deters slugs, so be sure to avoid using decaf.
7. Glue pennies on top of containers or place them around your garden. Just like the copper coils, when slugs get near them, it sends an electrical charge.
8. Slugs stay away from vinegar, so spray it in their paths (not on them). However, it is an herbicide, so be careful not to spray around plants!