I set my computer to print in "Grayscale" and I've told it to only use the black ink cartridge, black is cheaper than color and if I do need to print in color I can just temporarily change it back. If you don't change to "Black Ink Only" it can still be using ink from your color ink cartridge even if you are only printing black.
I've also changed my printer to print "Fast Draft", it still prints out great for coupons or just everyday things you need to print. If you are doing an important project or printing photos you can go back in and temporarily change it. Changing to Fast Draft saves a ton of ink, you'll be amazed how much longer your ink cartridge lasts.
Another thing you can try is changing your default font to a font that uses less ink. Some fonts are fatter and have more details so it uses more ink to print. I don't know how much ink you can save, I haven't been able to tell a difference in how long my ink cartridge lasts but you could try it, every little bit helps right?
A few fonts that use less ink:
Times New Roman
I haven't done this yet (just cuz I'm lazy) but you can download a free font called Ecofont that was created to save ink, it has tiny holes in the font to use less. It's supposed to be similar to Arial, Arial is a common default font but uses a lot more ink.
If you want to find deals on Ink Cartridges you'll want to check eBay, I've been able to score some good buys, the inventory changes so check back often.