All great advice there. The only thing I can add is mix up water and dishwashing liquid in a 5 gallon bucket. Pour that soapy mix where you usually find crawlers and it'll make them come up!!! I haven't done it but have had plenty of people tell me it works very well!Water.. Water... WATER!
Water as large an area as possible preferable in an area with lots of trees/shrubs. IMHO worms seem to like areas where there is decaying leaves and such and will hold a much greater amount of nightcrawlers then just a grassy area.
Wait till well after dark and keep lights to a minimum. When I go worm hunting I carry a flashlight with batteries that are weak and almost dead. Also walk as slowly and quietly as you can.
Once you spot a worm, do NOT shine the light directly on them. First thing you need to do is figure out which direction the worm is facing. You will want to grab at the end that is nearest to the hole in the ground. You will need a firm (but not TOO firm) hold on him. Apply a steady pull on him and you might feel him pull against you slightly. Don't pull him in half, rather, wait for him to relax his pull a little then pull him a bit harder. At that point he should come out of the hole easier.