When you move you usually spend a bunch of time tracking down boxes to transport all of your stuff. My wife and I have hit up grocery stores and other local retailers for boxes that they would otherwise crush and throw away.
U-Haul offers a box exchange resource for prospective movers. It is a simple forum that is split up geographically to help you trade new or used boxes. So rather than running around to retailers you can check the forums for your area and see if there are any boxes available in your vicinity.
After you move, rather than throwing away your boxes, you can post a message to the board and give them to someone who can use them again. If you can’t find boxes via the U-Haul forums, you can also try FreeCycle, an Unclutterer favorite.