One natural slug predator is garter snakes. I have recently turned up four garter snakes which might give a clue for attracting and keeping them:
1. Under a 1 foot diameter boulder which was half buried. I would guess this was a hibernation site.
2. Between the large stones that surround my
compost pile.
3. Under a 1 foot high pile of loose cut grass (cut with a scythe).
4. In a loose section of a pile of wood chips.
I'd try the "mini haystack" method first. I was not trying to attract snakes, I had recently mowed a small field with a scythe and was raking up the cut grass to use for mulch when I found the snake under one of the larger mounds of cut grass. I think it is important that the cut grass be long, and loosely stacked. I don't think a pile of grass clippings from a lawnmower would provide the same micro-environment.
Here's a video of some people using scythes, you can see how they leave rows of cut grass:
http://www.youtube.com/watch?v=gL2_chKPWjE However I am not that good, my rows are very sloppy
It would be interesting to do some experimenting with mini-haystacks to see if they do indeed attract garter snakes.