Bird Feeders by Droll Yankees
Droll Yankees sell a whole line of motorized and battery-operated squirrel-proof feeders that range in price from approximately $80 to about $120. Their basic model uses a battery-operated perch that will spin when it senses the extra weight of the squirrel. The idea is that the squirrel will tip or fly off. Though entertaining and fairly effective, the Droll Yankees' models need new batteries every two to three months.
Provide a Distraction
Probably your best defense against squirrel invaders is to simply give in and feed them too. Try buying block bird seed, which is heavily condensed into a compressed block, and placing it on the ground. Instead of putting in all that work too conquer your feeder, your squirrels will likely be content to nibble away at ground level.