I've had very good luck with one of those metal frisbee-shaped dish things mounted about halfway up the post on which I have the feeders mounted. The squirrels don't seem to have the ability to get past it, so they graze under the feeder and under the oak trees in the woods behind me.
I have 3 types of woodpeckers that show up. When the feed, they scatter plenty of seed on the ground below for the squirrels to eat. I have at least 1 pair of Red Bellied WP, Hairy WP, and Downy WP, and last year, a juvenile red-headed WP showed up a few times. Hoping it returns with a mate.
I have those woodpeckers as well except the red-headed. I do have Northern Flickers with the gold tails.