Slate's Farhad Manjoo has some great tips for outsmarting the greedy, lying sensor in your printer that wants you to change the super-expensive cartridge before the ink runs out:
This guy had also suspected that his Brother was lying to him, and he'd discovered a way to force it to fess up. Brother's toner cartridges have a sensor built into them; OppressedPrinterUser found that covering the sensor with a small piece of dark electrical tape tricked the printer into thinking he'd installed a new cartridge. I followed his instructions, and my printer began to work. At least eight months have passed. I've printed hundreds of pages since, and the text still hasn't begun to fade. On FixYourOwnPrinter.com, many Brother owners have written in to thank OppressedPrinterUser for his hack. One guy says that after covering the sensor, he printed 1,800 more pages before his toner finally ran out.