Clear any rubbish up thats laying about, if you have a tidy garden they will go where there is an overgrown one.
The problem is telling them that its nicer elsewhere.
But tidy up, use pellets, collect as many as you can eventually you will get to an acceptable level. I found a pile of snails all huddled together about a month ago...crushed the bloody lot...there are plenty left for the birds before anyone starts.
If using the blue pellets make sure you clear away the dead snails on a daily basis so that other wildlife (such as frogs, hedgehogs, thrushes) doesn't eat the poisoned snails and get poisoned as well.
It's a U.K wide problem, thanks to warm and wet days and nights, perfect weather for slugs and snails. Buy the pellets with animal repellent impregnated in them, to give other wildlife a bad taste if they try to eat the dead slugs. A beer trap works well, if you can bring yourself around to pour good beer in to the trap Here in Devon we have had no frost this winter, consequently I have greenfly on my hosta's, slugs and snails queueing up to enter my garden, blackfly on my osteospermum, and a whole host of pests, and it's only May!