Slugs really can be a bad pest in a garden.  Here are a few suggestions that might help.  You can line the sides of your garden boxes with copper strips or copper wire.  You can obtain copper wire by finding old telephone wire and stripping it.  This may help as it gives the slugs a shock.  Now, another and somewhat gruesome method of ridding your garden of slugs is to go out late at night with a flashlight.  Take a container of salt water and a large hat pin.  You can then either stick the slugs with the hat pin to pick them up or you can pick them up with your fingers (you may want to wear gloves), then put the slugs in the salt water and discard.  You will notice after doing this for a few weeks that the slugs start getting smaller and smaller because you are getting all the adults and your slug population should diminish.