I've had great success with a couple of barrier methods:
a) copper tape stuck around the sides of my raised veg beds (about 2/3 of the way up seems to work best, against slugs but not snails).
b) Vaseline smeared across the tops of my troughs and pots.
The first seems to have kept the slugs away so well that I am now keeping my lupin seedlings in spare space in the beds - slugs love lupins. I started off with twenty, but the little blighters got almost all of them.
Since I moved the three surviving ones, no problem.
The second was a tip from a neighbour, and I'm sure it's the only reason I still have any basil or coriander! I had previously never managed to plant either of them out without them getting munched to the ground within a day or two. The trick is to make sure that no grit gets into the vaseline layer, as it works by making the surface too slippy for the snails. Of course, you also need to make sure that there are none in the area you are trying to protect to start with, or they will not be able to get
out, and will just munch the available food supply while they are trapped. For the same reason, you might have to keep checking the area for a bit after you've done it, in case of newly hatched slugs from eggs already in the soil.
It also seems to help to keep plants in pots or troughs well supported with sticks and such, so that they don't hang over the sides to the ground and give the slugs/snails a 'bridge'. If you do the smearing while the surfaces are dry and clean, then you don't need to bother again for
months, as vaseline doesn't get washed away by rain, it just sort of soaks into the pot but still leaves a slimy surface.
Good luck
