use a small-mesh produce bag, fill w/ stuff they particularly like, say chopped up melon - tuck it along one side of the bin - works w/ just cooled material from a hot compost bin too

another way is to lay some fiberglas fly screen on the surface and put a bit of wet shredded paper on top along with fresh food ... they'll wiggle up thru the screen to get the feed and you can just lift the screen off after a while

easiest way to get the critters out is to heap up a mound and let it sit 5 min - start raking off the surface and you'll find the worms go toward the bottom center ... when you see many worms wait a few more minutes then rake off some more - eventually you end up with a small mound that's pretty much all worms ... easy to pick'em out if you're harvesting the worms, or just use that pile to restart the bed if you're taking off the castings

worms won't move out of material that you don't see as providing food because they can continue to 're-process' the castings for quite awhile before they look for greener pastures, you hafta remember they're actually eating the decomposer bacteria, not the veggies

Bill