Take it from a man who has moved more than a dozen times, the best place to get great boxes for free is from grocery stores at 3am!
Depending on what you are shipping, the key is to double box the blasted thing! Always double box and your buyer will love you for it! Secondly, make sure that the inner space is about 4" on all sides as well as top and bottom. Then get a sheet of 4' x 8' x 1" styrofoam insulation from Home Depot (or the like) for a couple of dollars. Break off pieces of the foam to fill all of the 4" interior spaces, and make sure to stuff it tightly so that the foam presses against both your inner (product) box and your outer (grocery store) box. If it's packed too loose, the foam will be of little protective use. Stay away from the packing peanuts (there was a thread on this yesterday) because they are useless for heavy items like amps.
If you are shipping large speakers and don't have the original boxes, then shame on you! (It really helps for warrenty purposes to send items back in their original packing materials). For all other items, including small speakers and anything that would fit in a standard rack space, the double boxing with foam works every time.