First, ink cartridges can be refilled and reused several times before they're completely worn out.  Many stores will refill your cartridges for you, check your area for who offers this service and have your cartridges refilled if they're still usable.  When the time comes to get rid of them, DON'T put them in your recycle bin!  There are specific ink cartridge recycling drives, and these are the best way to get rid of your old ink cartridges.  Staples offers a program that will send you a prepaid shipping envelope for you to send in your old cartridges.  You still have to pay for a new cartridge of course, but this service lets you recycle from home instead of hunting down a recycling drive.

Also, if you purchase new ink, most companies now provide a bag to put your old ink in that can be mailed to a recycling center! It's free for you and you just drop it in the mailbox.