The most elaborate "squirrel proof" feeders use a small solar panel to charge batteries that deliver a gentle shock to the squirrel. But the most effective - and entertaining - feeder on the market today is one that uses a ring perch around the bottom of the feeder. When the squirrel grabs the perch, his weight causes internal contacts to close, starting a battery powered motor to spin the perch ring. Squirrels will usually hang on for two rotations before they spin off. Squirrels are very resilient and are not injured, and after a few experiences with the "ring," they will not touch it. Fun stuff!
Which ever type you select, make sure that the feeding stations are metal and the top of the tube feeder is metal as well. Squirrels will chew through the cheap, all plastic feeders in no time, and you've wasted your money. A quality feeder will last for many years.