Best way to extend the life of flower pots?
I've been told that sealing at least the inside of your flower pots (terra cotta, etc.) will extend the life of them by protecting them from the freeze/thaw damage of winter ... some people say they use Thompson's water sealant, some say oil-based polyurethane, some say water-based polyurethane, some say a 10:1 solution of PVA (poly vinyl acetate).
What is the best way to get more life out of my pots? I'm replacing them far too often. I have to store the unused pots outside, but I also have some plants that I keep in pots outside over winter.