[Barry Lewis]

I thought that I would share my new idea for protecting those crops without any need for harmful chemicals. The idea behind this design is simple and effective, so there is just a description of my design to illustrate this simple deterrent....
There is a little cost involved, but we'll worth the outlay:-
Items required for each plot....
4 lengths of guttering
4 gutter corners
1 bag of sharp sand (dependant on gutter length)

Method:
1/ Make up the frames ,either square or rectangular to the perimeter size of your plot.

2/Lay a bed of sand to which the frames will be placed.

3/Place the frames upon the sand and fill with water, then press the gutter frame into the sand to attain a reasonable equal depth of water...

As you can see from the principle, the critters will need to cross the water filled gutters to reach your lovely veg...
I know it's overkill, but many may wish to sit the frames on a concrete bed for longevity...

[Goosegirl]

OY! You've pinched my idea (see another post from me about slugs and snails)! Great minds think alike. 

[Spr0ut]

But some slugs can burrow and some live in the soil and come up to feed.

Sorry - otherwise I like the idea - better than my coffee grounds which have proved useless.

[mumofstig]

Yes, slugs live in the soil like worms - so it won't stop them 

[yorky]

The idea presumes that slugs live somewhere other than on the plot and only venture on to the plot when they want to feed. As others have said, the slimey little critters and their eggs are always present in the soil. Sorry to be negative but I don't think that a barrier makes much difference.   

[DD.]

Black keeled slugs (Milax spp.) live underground and tunnel into potato tubers and bulbs. In some gardens these slugs can damage a large proportion of the tubers of maincrop potatoes

So, not even the most sophisticated barrier will stop them.