With some HP DeskJet printers, you need to fool the printer into thinking you have replaced the old ink cartridge with a new ink cartridge instead of a refilled ink cartridge that you filled yourself. If the printer keeps telling you to replace the ink cartridge or the ink level still reads empty, you can reset the printer so that it will recognize...
How to Reset a Canon Ink Cartridge | eHow.com
Let's face it, ink cartridge manufacturers like to give you "low ink" or "empty" warning messages even when you know that can't be possible. However, there is an interesting hack that may work on some brands that will trick your computer into squeezing every last ounce left in it.