Slug pellets work great but they can kill many birds (especially thrushes which eat slugs/snails) and are sometimes eaten by dog/cats with serious or fatal results. There are supposedly safer chemicals available but they tend to be less effective. Many people use "slug pubs" to great effect. This is done by sinking something like an empty yoghurt pot in the soil and putting a couple of inches of beer/lager in the pot. Cover with a loose fitting lid and slugs will flock to this treat. Thing is they very quickly drown in the beer. A more high tech treatment is to use a parasitic nematode. These lovely beasts lay their eggs in slugs causing a particularly gruesome death. You can buy these online and in some garden centres. Lastly and of lowes cost you can go out once dark with a torch whereupon you'll see dozens of the blighters. Kill on sight and within a couple of days your garden will be near slug free. Cheap and environmentally friendly!