So you want power generating pants that are still comfortable? Then you are going to need piezoelectric nanowires. These are not readily available, but the gang at NASA are working on it.
Why are they bothering? Well, self-powered spacesuits would be quite a coup. The nanowires are not easy to make, but researchers at the venerable institution have recently figured that moving the operation into outer space makes things considerably simpler. Gravity be damned!
Of course, this is still in the early research stages so it may be a long while before you get yourself a pair of self-powering jeans. A blogger can dream though, right?