Boxes, for sure. Especially at this time of year, bike shops will have plenty of them to give you, complete with the packing materials. (New bikes come with round cushiony material around frame tubes -- looks a bit like pipe insulation.) Remove pedals and front wheels, loosen stems and turn handlebars sideways. and -- if a bike doesn't fit snuggly in a box -- punch holes in the sides of the box and run long zip ties to hold the bike in place so that it doesn't bounce around while being moved.