I have had best luck over many years of bird feeding with a 2-pronged
First, a squirrel baffle consisting of a large tube over a smaller
tube. Large tube needs to be 4+ inches in diameter, about 18 inches
long, and mounted to small tube with a bracket that will allow the
larger tube to swing back and forth (think of a coffee can inverted
over a broomstick to get a visual of what I'm talking about).
Squirrels can climb the small tube but can't get around the larger,
unstable tube. Of course, this assumes feeder is mounted high enough
from the ground and far enough from the roof to preclude squirrels
jumping onto it.
Second half is to provide squirrel feed elsewhere, far from the bird
feeder. In my experience, there's little they like as much as
birdseed, but corn and other wildlife feed offered in an easily
accessible spot is often enough to swing the balance away from trying
to defeat a more difficult birdfeeder setup.