It's barely worth the trouble
Most of them have killer chips, which basically stop the toner cartridge from working when it's empty.

You have to replace that as well, I worked out refilling costs about 2/3 the cost of a new one, and is messy.

Cheapos aren't good either. Last week I ordered a non-OEM one for my Samsung colour laser printer, it arrived with half the toner on the outside of the cartridge, so went straight back to Amazon.

[edit] If you feel brave, Google 'killer chip [your printer model]' and look at the youtube videos. Quite often some mad Russian has worked out how permanently disable the print count function, so you can just top up the cartridges with cheapo toner indefinitely.

Normally requires soldering wires onto chips, so it's not for everyone. Obviously kills any warranty too.
(username failed moderation, Thu 1 Mar 2012, 8:16, Reply)