YSK: You can "sharpen" razor cartridges by rubbing them on a pair of jeans and get MONTHS out of one cartridge, if not more.

I tried this trick a few months ago and was pleasantly surprised at how my blade pretty much sprang back to new. But, since I work from home, I only shave once a week and I totally forgot about it.

Then I watched this video:

https://www.youtube.com/watch?v=dnyIU17KTwY

EDIT: you only need to watch the first 5 minutes of the above video.

The guy has been using the same blade to shave his head for 2 months now.

Other sources:

https://www.youtube.com/watch?v=OXaK9yr1D5c

Just a general howto video

https://www.youtube.com/watch?v=h-4bhVpfxJM

USB Microscope video of the blades before and after, showing you that it actually works.

Supposedly soaking your blades in mineral oil can make them last years. I haven't tested this theory yet. But mineral oil is cheap, so i think it's worth a try.