Gardeners question time mentioned a great slug repellant last weekend and I've just tried it.
Crushed oyster shell.
I just put out a 1 inch wide mound of it around the edge of my veg bed and collected a few snails to test, they wouldn't cross it. I've never seen a snail turn so fast so I assume the slugs won't like it either.
If you live in an agricultural area like I do (cumbria) then you can get it from an agricultural shop for around £7 for 25KG. I used about 8KG to put an edge all the way around a 2x2m veg bed.
It's high in calcium so it's generally used to make chickens eggs stronger so I'm not sure what the effect of having a rich in calcium edge around the veg bed will be.
Early days but I'll report back at the end of summer about how well it worked.
If you don't live rurally you can get it online from a place like this:
http://www.valupets.com/pages/html/chicken_feed.html
It will raise the ph level though.