I find that to get the best results from this you need to first scout the area for field signs. Many animals will have regular routes, look for slides under fences or wholes in hedges. You may also get lucky and find animal hairs caught on the bottom of the fence or hedge. Droppings can also give you a good clue to what it passing through. Birds like sparrowhawks will have regular plucking posts where they strip their prey so always look at the base of fence posts etc for feathers or pellets containing the animal remains.
After that its just a case of sitting hidden and quiet and hope for the best!
Scott