This works pretty well for me in chapparel with rabbits and ground/rock squirrels, but not so good with tree squirrels. First of all, find their den/warren. Set up your blind about 20 yards down wind and wait. Don't fire until they clear their holes by at least 5 feet for a .22 LR or ten feet for a high powered air rifle. With ground squirrels, you probably won't be able to set up downwind, because they make their dens in enbankments, but luckily, they don't react to scent as much as rabbits do.

A good accesory to take with you when hunting ground squirrels is a squirrel distress call. Do the continuously timed squeek (e.g. terrestrial predator. If you use the broken rhythm of an aerial predator distress call they'll all scatter in seconds) and you'll make 'em freeze in their tracks. A hawk call serves the same purpose for rabbits.

Another thing is ALWAYS AIM FOR THE HEAD. I have shot squirrels clean through the heart with aluminum arrows and they still managed to drag themselves 20 feet before dying. This is bad news if it'a near its hole.