I intend this question is more of a community wiki than anything else.
Recently I had a problem with my HP Photosmart CS5300 series inkjet printer. I've been printing 99% black draft text documents for about a year. However, the printer was reporting that ALL the cartridge levels were going down. What? how strange I thought. Then I discovered that these levels were actually an estimate and the printer wasn't reporting a true ink level.
What happens when the estimate reaches zero? I discovered the answer to this at just the wrong time. It stops printing anything and just shoves out totally blank pages.
So, Googling around I discovered this is actually a very common problem with many brands of printers. It seems that the industry has decided to protect their investment in ink and make the cartridges expire so that refilling them will not work.
However, it seems there are some solutions to this problem. I don't care too much to refill my cartridge I personally just want to use the ink that I paid lots of money for. This is what I did for the HP CS5300 printer.
On the printer itself, press the settings button (on the right hand side next to the help button). Select Tools. Now select to print a printer diagnostics page. I just so happened to also select to a clean print heads as well. One of these or both may be needed. Once I did this, I found I was able to print normally even though the printer says that there is no ink. Well that fixed it for me.
What methods have you found to get the rest of the ink or toner out of your printers?