1) If the color of the text is not important - once the black ink runs out - try printing in dark purple - this is a mix of red & blue. The color of the best mix that uses NO black is #660066 - This mix should run your red & blue down evenly. On things like my Paypal invoices & printing up labels - it really doesn't matter what color it is printed. Next, try dark green - that color is #006600 - guesstimating on the color wheel is also reasonably sufficient. Just because you run out of one color doesn't mean you have to buy all new cartridges.
2) If printing text in black - it's best to have an old Apple LaserWriter or an HP Laser printer to print just black text - a laser printer can print roughly 4000 pages of black text. A laser cartridge costs $50-$80. An inkjet cartridge can print roughly 250 pages of black text and cost roughly $25 each.
3) If you have the time - isolate & crop your text or pictures better - removing any unnecessary color or cropping out people that got in the sides of your picture can save a TON of ink
4) Don't use Epson or Lexmark printers - both are ink hogs.