A damp plastic garbage bag laid on the ground at night will also collect slugs for morning disposal. In truth, slugs far outnumber even our best estimates of their population. Handpicking does help, particularly if done in early evening at dusk as they come out to feed. Of course, all slugs and snails will feed during dark, moist days. Collect into a container with a secure lid such as a mayonnaise jar with soapy water in it.
Live slugs can push off pop-off lids or eat their way out of cardboard containers.
Other than hand-picking, consider some barriers as garden management techniques. Some gardeners find that a strip of cultivated dirt, bare ground, about 3 feet wide, scuffled often with a hoe, and kept dry, helps to repel slugs. Various sources in home gardening magazines have recommended sand, diatomaceous earth, cinders, wood ashes, lime, and eggshells, most of which work only intermittently and not at all when wet. Copper barriers have been tested in the Northwest for a number of years and show effectiveness. A strip of copper sheeting stapled to a raised bed board can work if no vegetation dangles over it to provide an alternative slug route, a sort of trapeze for them to hoist over the copper barrier.