Depending on the law in your area, you may want to consider using poison
bait. Squirrels don't give a damn about rat bait, they ignore it. In
california you can buy this stuff:

http://www.wilcodistributors.com/pest_control.htm

It's sold at home depot and lowes, it's usually stocked on the bottom shelf
of the pest/insecticide shelves.
Works fairly well, just make sure it's placed in a proper enclosure, so pets
and birds can't get at it.

-S