I had a tough pair of rims that were much improved by going from the regular cotton rim strips to a thin polyurethane one. A thinner rim strip allows the tire to sit lower down in the center of the rim. You have to look around, though. There are also poly rim strips for sale that are too thin to do their job.
And like eippo said, you keep the tube valve open and push the bead down into the center of the rim on the side close to you, then keep tension on the bead as you slide your thumbs toward the opposite side. The tension holds the bead in the center of the rim. But even that doesn't work with impossible rims, which do exist.