Pour vegetable oil on the glue around the animal. I have used this successfully to free snakes and lizards and even a wren that got stuck on one of these things. I HATE these glue boards, and find them horribly cruel and nondescriminating; they catch any small animal that wanders across them, whether or not it's a pest. Our school has to use these to avoid poisons, which can hurt children, so that's why I have to know how to remove animals that get stuck. They are mainly put out for cockroaches, but I seldom find any cochroaches on these things. I won't let the exterminators put them in my classrooom. I have many native spiders(harmless species, as are most spiders) that live in my classroom, and I've never had a problem with cockroaches, for some reason.
experience removing animals from these *&^% things