A lot of people simply throw out old ink cartridges after they are empty.
Aside from being environmentally unfriendly, it's also not cost effective! The three major office superstores will give you $3 per cartridge, typically, though each store has different requirements on brands, etc.
Staples accepts empty ink cartridges from Dell, HP, and Lexmark, and will give you a $3 coupon off any future purchase. You can use it towards anything, and can use up to 3 per transaction.
Staples Cartridge Recycling Program info on Wikipedia
Office Depot also accepts ink cartridges from some companies, and will give you a $3 coupon off future purchases:
Office Depot Cartridge Recycling Program info on Wikipedia
Office Max accepts ink cartridges as well. The last time I did it, it gave me the option of $3 off any future purchase OR a free ream (500 sheets) of printer paper. I went for the paper.
Office Max Recycling Program info on Wikipedia