Breaking the bead is the hard part. If you can get the bead to pop by driving over it, then it's really pretty simple to get it off with a couple of tire irons, or even with something else like jack handles, if you don't care about the tire. A small sledge hammer is handy too. Try to wear heavy boots with tough soles, because your feet will be contributing.
Get the bead to pop on one side (using car, bead breaker, large tire hammer, or whatever), then turn the wheel over, and make sure you can either push or stomp it off on the other as well. Both beads need to be freed, although the back side can be allowed to pop back on again. I have a hand bead breaking tool that makes the job fairly simple, but you can do it by driving over it, or sometimes with a jack placed well, though this can risk tire damage.
Now lay the wheel down on the ground face up, and kneel or otherwise push on the face of the sidewall to get the bead down into the drop center of the rim. Hold it down, and 180 degrees from that point, pry the bead over the rim with a tire iron. Leave that iron in, and pry up next to it with another iron. If you do it right, the bead will stay where you pried it and you can then proceed to work your way around the rim. Once the bead is about a third of the way off, you can usually just pull it off.
Now stand the tire up vertically and make sure that the bead of the back side of the tire is in the drop center. At the top, put your tire iron in from the inside of the wheel, hook it on the rim, and pry the bead over the front edge of the wheel, pushing it over as far as you can. The iron will be pushing down the sidewall as well. Hold it as far down as you can. You will now have the inside of the back bead exposed, and you can whack the rest of it over the edge of the rim with the hammer. After it's over a certain amount, the wheel will simply drop to the ground.
One of these days I should shoot a sequence of pictures to illustrate. Back in my wasted youth, when I owned old Peugeots, which had solid center wheels (no fit on machines), I learned to do this, and have been putting skins on and off for years. Once you have the knack, you can do a tire in about a minute with two spoons and a hammer, as long as you have a reliable way to break the bead.
