Hi! That is a good question! Squirrels are TOUGH! I found that mixing cayenne pepper, or any hot red pepper with my seeds helps. The birds don't have much for taste buds, so it doesn't bother them, but the squirrels don't like it. I've still not figured out a way to keep them out of my window boxes, but I know that sometimes they dig them up because the roots are moist, and they're thirsty. I don't mind them drinking out of my birdbath at all, so keeping it filled does help some.
Outwitting Squirrels: 101 Cunning Stratagems to Reduce Dramatically the Egregious Misappropriation of Seed from Your Birdfeeder by Squirrels is a great book by Bill Adler Jr. Several of the 'stratagems' are jokes, but there are actually a lot of good tips and hints.
The above is the amazon.com page, and I see that there are 182 used copies for sale, with the cheapest going for $0.01 plus shipping. I assume that some folks just give up.
Here is a list of other books purchased by people who bought Adler's book:
If you aren't interested in buying, you may have a good list of books to ask for at your local library.
Anyone else have any suggestions?