Unless you're printing out an important document for a client or presentation, use the draft setting in your computer's print options. Printing in draft mode will not only use less ink, but it will also print more quickly. You won't even notice the difference in quality.
You also don't need to print in color every single time. Color ink is more expensive than black ink is. Change your printer properties to print in grayscale, and you'll save money on color cartridges.
You may not realize it, but most fonts are big ink wasters. Ecofont is a new type of font that conserves ink by omitting parts of each letter. The slightly holier font doesn't affect readability, but it can cut back how much you spend on ink.
When your printer sends a message that it's low on ink, ignore it. Manufacturers want you to run out and buy a new ink cartridge before your ink has run out. Don't do it. Have a refill on hand, but wait until the quality of your printed page starts to deteriorate before replacing the ink. You should be able to print dozens more pages, even after your ink has supposedly run out. You can also get more life out of each toner cartridge by shaking it up when it's running low.