When I had that problem, I bought squirrel food and put it on the ground, away from the bird feeder. I don't mind the squirrels, I just want the birds to get their food.
I posed this question on the boards a couple years ago. Someone told me to stir in a little hot sauce with the birdseed. The birds don't mind it at all, but the squirrels don't like it. Sure enough, it worked. No more squirrels hogging the birdseed and tipping over the birdfeeder.
We put vaseline on the pole that the bird feeder sits on. They try to climb it and slide down. You will need to retreat every so often. The feeder is also in the open.