A damp night , a flashlight and a plastic milk jug with the top cut off and filled with hot water. One of those thin disposable gloves is a welcome optional extra. Look under leaves as well, but they are usually fairly obvious, simply drop them in the hot water.
The board trick will work, but it takes time. Slugs and snails have a home they return to, from quite long distances sometimes, and it takes time before they adopt a new one.
I have never tried nematodes, but I am told they work.
Consider where they are coming from. Lawns and grassed areas are a major source, Give them something unattractive to cross to get to your beds. Anything within eight feet of a compost heap is in danger, site them well away. Pour boiling water down edges you can't get to and check under paving stones you can lift, they like safe hideaways.
Encourage predators, there are large ground beetles in most gardens, cut your edges with a steep slope so the beetle falls off the lawn into your bed and has to hunt there. I also have toads and slow worms .
The makers of modern slug killers call it 'organic', the active ingredient is an iron salt, which is relatively innocuous, but the iron causes liver failure in the slugs, I sometimes use a bit along the edge of the lawn where it joins the border. Remember it is a bait, it attracts slugs, so don't put it directly next to the thing you want to protect, use it where they will find it before they get there.
