My car bumper is a favorite - right foot braced on the trailer hitch, left foot in the stirrup, facing forward with left hand full of mane and reins, right hand on the cantle. Push the left foot in the stirrup up and forward, then let it swing back down and use that momentum and the handfull of mane to swing up into the saddle.
When I'm out in the hunting field, something that doesn't always occur to people is that if you can't get up on something, get the horse DOWN. I lead the Gracie mare into a ditch or sideways on a hill and climb on that way. If I'm out in the middle of a perfectly flat field (and that happened to me on the last Shakerag hunt - we had a longish check and my saddle pad had slipped), I lower my left stirrup about five holes and use the mane-and-stirrup swing technique, then raise the stirrup once I get back in the saddle. Works so long as you have a quiet horse who will let you readjust the stirrup without a fuss.