The best solution so far is to use double sided tape on the main speaker area. This stops the pad from sliding enough to fall off. Essentially it provides enough friction to stop them from moving around.
Does anyone know a source for obtaining replacement pads (somewhere between free and inexpensive, inclusively)?
First, you must have purchased an iPod. Two sets will come with those headphones.
Next, you must have a roommate who has purchased an iPod, but has ears which will not allow iPod headphones to remain in place. He will give you his iPod headphones along with the two sets of pads that came with it.
Finally, your headphones should go on the fritz while under Apple warranty. Apple will then send you a new set of headphones and pay for the return shipping of your old ones. The new headphones will also come with two sets of pads.
So..... I have six pairs of pads for two headphones...
; )