All good advice for squirrels so far. Especially the one that says listen a lot. You will hear them before you see them usually. If they see you and hide, just hang out and be still. They forget about you in about 5 or 10 minutes and start moving around again.
You will learn more by an afternoon in the woods than we could ever tell you here if you pay attention. Listen to all the sounds, keep watching the trees, trail and air. Watch the birds take flight from the trees, perhaps a squirrel spooked them. Learn to identify the sounds you hear, it all works together in nature and I believe the animals warn each other. Squirrels have a distinctive shrieking type of alarm they sound. They have good eyesight so try not to make big movements.