Pair of jeans (old or new, it doesnt matter; just needs to have one of the pant legs in tact.
Shaver like gillete or schick brand razors or other disposable razors.
Before or after you shave (I prefer before so that the blades are dry, place your jeans on a hard flat surface; then run the razor up the pant legs about 10-15 times quickly; then repeat running it down the pant legs 10-15 times quickly. No need to press that hard, but a little pressure is necessary. In both instances, you want to point the top of the razor in the direction you are rubbing the shaver on the pants. In other words, don't "shave" the pants; point the razor the other way, so that the blades glide over the surface of the jeans and don't try to cut them.
The threads on the jeans then will very effectively both fix any tiny bends in the blades that inevitably happen and will also sharpen the blades on your shaver cartridge. For an already dull blade, you can sharpen it up pretty effectively by doing 50-100 swipes both ways to get it back up to "like new" condition, but only 10-15 times swiped both ways should be necessary to maintain sharpness.