We have a tried and true method of reducing our slug population each year, which works very well. We have about an acre of developed lawn and garden around our house. Each Spring, my wife and I go slug picking over about a two week period. We try and out do each other, as some days we can collect nearly 800 slugs (yes, you read it correctly)! After doing this, our yard is not quite slug free, but no issues in the vegetable gardens.
We usually dump the slugs across the creek we have in the back so we do not have to terminate them, and they do not come back.