Spuds, how do you keep the slugs out?

I find earlies and 2nd earlies are less damaged. This year, most earies harvested by 5th July and second earlies by 17th July. For maincrop, Carolus seem to suffer less than other varieties. I don't leave them in the ground any longer than needed. I usually plant a few Alouette and harmony maincrop which both suffer a lot more than the Carolus. These are all dug by early September at latest.