If you really want to make your bird feeder impenetrable to squirrels, you have to cut off every access path they have. That means two things: making the mounting pole impossible for them to climb, and placing the feeder out of jumping range.
When positioning the feeder, you may need to keep the feeder as many as 8 to 10 feet away from objects they could use as launch pads. This includes bushes and tress, of course, but other things like picnic tables, birdbaths, etc. Also, be aware that if there are branches above the feeder, the squirrel can drop down from seemingly insane heights to land on the feeder.
For the pole, the best bet is to use a baffle of some sort. All of the baffles should be "wobbly", so that if they get a grip on one edge somehow, it falls down, allowing the squirrel to drop instead of continuing to climb. Avoid plastic baffles, and use metal ones. Plastic ones can be chewed...squirrels have incredible teeth. And even if they don't chew through the baffle, they can scuff it up enough to get a better grip for climbing. The best type of baffles are the ones that are more like a can than a saucer. The squirrels climb up inside, find the way blocked, and back down. If they try to go on the outside of the can, it pivots and they slip off.