I have done it before. I first pushed the tire away from the rim. Then I used two large screwdrivers, a large pry bar, and my wife to help hold it.
I believe it would have been much easier if I had used some grip pliers to hold the tire after I had pried the edge of the tire over the rim of the wheel. (This would keep it from slipping back over the rim again.) Then I would alternate using the two flat bladed screwdrivers to pry the remaining tire over the rim.
If the tire is just flat, you can slip a tube inside without removing the tire. (You will have to remove the valve stem.)
Andy, February 2008