I have sucessfully cleaned my Canon i560 printheads as follows:
Remove the print head and handling it gently place it in a bowl of warm (not boiling) water mixed with 360 ml (a small bottle worth) of printer cleaning fluid, (the kind that comes with ink jet refill kits)
If you haven't got any of this, then a small amount of Fairy washing up liquid in it, might work but the printer fluid is better. Leave it soaking for 24 hours, well away from the kids and the cat, and you should return to a bowl of dirty water as all the ink should have drained.
Now this step is critical. Do not attempt to take a apart or rub violently. but very, very gently using a soft cloth, drain and dab away the excess ink seeping from the head. Be sure that your print head is dry before you put it back into your printer.
When you are sure that it is dry, put your cartridge back in your printer. You will need to do the normal cleaning tests, - a print nozzle and a clean head test. Hopefully it will be back to normal.
I once bought a new Canon i560 because I thought my old head was broken. I completed this cleaning and my old one worked like new.
I hope this will work for you.
David Murray
April 2006