Weatherstripping is GREAT for cracks up to ~3/4 inch or so.
Bigger than that you can improvise... that dark grey foam pipe insulation is easy to cut into whatever size chunks you need, and cram it into cracks.
If the closet is big enough for you to stand in it, get in there with a roll of duct tape, turn off the lights, close the door, and go nuts. You have to use a few layers though- the stuff is not entirely light proof. Plus it stands up badly to heat, so you have to replace it every so often.