Here's a perfect example of one of the very few times I've had to do this. The last half of this vid shows me climbing a hill with grades up to 25% on my single-speed 16" wheel Dahon Da Bike folding bike. I had to "tack" the hill on the steepest parts in order to make it up.

Be forewarned, there is very heavy breathing in this video due to the effort involved. Also, I crash at 1:13 when the handlebar latch comes undone when I attempt to climb out of the saddle. This causes me to laugh uncontrollably, even after resuming my climb.

http://www.youtube.com/watch?v=ow_8b-7AfN4