To demonstrate the ease, I chose the most difficult combination I have: Birdy wheel (18") and Marathon Plus tyre.

Step 1: FULLY deflate the tyre.


Step 2: With valve at 9 or 3 o'clock, squeeze the tyre beads together while sliding hands forcefully from to pto bottom.


Step 3: Sliding down while squeezing beads to the center of the rim well...


Step 4: When the hands reach the bottom, invert the wheel and strat pushing the beads over the rim wall. In this pic, the bead can just be seen proud of the rim wall.


Step 5: Continue pushing both beads over the rim wall until about an 1/8th is done.


Step 6: Now grip the section that has been pushed past the wall and just pull firmly away from the rim and the whole tyre will slide off at once.


Here is a crappy video:
http://www.youtube.com/watch?v=7xZ6Lybtc8Q

I don't have very strong hands at all. In fact I think they are below average.