the short answer is really you cant stop them being in your garden. however you can sprinkle sparingly slug pellets [1 pellet per square metre apparently is the new thinking of use instead of a load of them.]

I have also heard putting a small cardboard box on the ground gives them somewhere to hide during the day. Beer traps/ half skins of citrus fruit also works.

They can then be killed or relocated but a good distance away. Don't just pop them over the fence as they will come back.

One thing you can do is invite birds into your garden with a cool fresh water bath. They'll gobble up a lot of snails, slugs and whatever else is around.

The time to effectively reduce the population is winter. Search about until you find some hibernating - they tend to cluster together, often cemented into clumps by their own slime, which dries rock hard They are easy to pick off and remove. What you do with them after that is up to you, but best not to leave them in the garden!
Not much use to you right now but it works long term.
(Somewhat ashamed to admit I once put a supermarket carrier bag at least half full, into the dustbin. (I counted 200...) Well they were all fast asleep...) Not much help right now though...

That's a good tip, Yorkslass. I've never bothered to look for them in winter but it makes sense to do so.

I'm sorry but slug pellets are very damaging to other wildlife. Please don't use them.

Well how do I get rid of the slugs and snails please Merliinbabydc ?

You could use beer traps if you aren't squeamish about cleaning out the trap each day. Get a shallow container (the base for a pot is ideal) and half fill it with beer or lager and place it where you have the most snails and slugs.

You can catch quite a few that way.