Is your printer lying to you?
We all know that laser and inkjet printer manufacturers make their money on the "give you the razor, sell you the blade" model. Is it any surprise that your printer might tell you that the toner cartridge is empty, when, in fact, you can print several hundreds more sheets before replacing it? In the good old days, you merely took the cartridge out and gave it a good shake and kept printing . Now, the manufacturer's have gotten smarter and put chips into the cartridges that tell us when the cartridge is "empty".
On, Fixyourprinter.com, one fellow claims that he got 1,800 more pages after he hacked his Brother printer by covering the sensor with electrical tape. This let him continue printing even though the printer claimed his cartridge was empty. Your mileage may vary. The site also offers references for error codes, printer drivers, etc.