1. There are several kinds of squirrel proof birdfeeders on the market. Whiile this may not solve the problem entirely, its a good place to start.
2. You can make or purchase a squirrel baffle. You can purchase a squirrel baffle in different shapes and sizes. (cone, cylinder, dome shaped.) If you are protecting a feeder on a pole - Drill a hole in the center of the bottom of a large can or bucket and stick the feeder pole through the hole. Mount the can a few inches under the feeder itself. This will prevent squirrels from climbing up the pole to your feeder. Try greasing the pole or spray it with Pam. The oil will make it difficult for squirrels to climb up the pole. If your feeder is hanging from wire, string the wire with empty film canisters (lids on), that will spin and dump the squirrels off.
3. Try adding a tray to your bird feeder. This will keep the majority of the seeds off the gound. The more seed there is on the ground the more likely you'll attract squirrel. Don't mix bird seed. Birds tend to throw seeds they don't like to the ground
4. Set up a separate feeding station for the squirrels far away from your bird feeder. Squirrels love dried corn and nuts