What's the best way to recycle used print cartridges?

Instead of throwing them out, you could try your hand at refilling them. http://www.wikihow.com/Refill-and-Reuse-a-Printer-...

There are several government agencies and not-for-profit organizations that provide recycling services for laser toner cartridges. You just have to pack the laser toner cartridges properly and safely and then inform your local organization, which will come and pick up the cartridges and recycle them for you. You also can drop them off at a drop-box location for one of these organizations.

Packing Laser Cartrdiges

Before your cartridge is picked up or before you drop it off, you need to pack it safely. The easiest way to do this is to place the old cartridge that is being disposed of in the package the new cartridge came in. If this is not an option, you can roll the laser toner cartridge neatly into a newspaper or bubble wrap. Most laser toner cartridge companies have clear instructions for how to do this.

I've seen a box to place your used ink cartridges in for recycling purposes at the "big box" stores like Walmart. I surely don't recommend attempting to refill your own ink cartridges to re-use. It's not only messy, but can void warranties of some brands. Good luck!