Change up the font.
Smaller, thinner letters equals less ink. Every time you print a big document, switch up your font to a thinner typeface to use less ink overall. Switching your font size to something one pixel smaller will also have a big effect on your ink use and a small effect on your doc.
For extra credit on saving ink with fonts, install the Ecofont typeface. This completely free font is designed with tiny holes in each letter and symbol—invisible at 30-point size and smaller—saving you ink a few drops at a time to make the whole cartridge last longer. Get it here.
Print only what you need.
If you're serious about making your ink last a while, take some time to mess with the print settings in your software and web browsers. If you don't need page numbers or the URL in the footer of every page you print from the web, turn that functionality off.
If you only need to print a small section of a web page, highlight only the text you want before you hit "print"; you should end up with a printout that's just what you need. Or use a service like Print What You Like or GreenPrint, which both allow you to customize content when printing from the web.
Switch up the colors.
Black ink runs out first, it's just a fact. So the next time you're printing something that doesn't need a professional polish—like concert tickets or a recipe—change the color of the text in your document to blue or magenta. That way, you save all the black ink for important stuff.