I read this in one of my trade magazines, I have needed this trick before, :)
Tips and Tricks,
The next time your printer's ink cartridge runs out at an inopportune moment, grab a hair dryer. You might say, "This is no time to fix my hair. I have to print the final three pages of this contract." Relax. Usually, your cartridge has enough ink for a few more pages; you just need to coax it out. That's where the hair dryer comes in. Remove the cartridge from your printer and blast it with hot air from the hair dryer for a few minutes. Replace the ink cartridge while it's warm, and try to print those last important pages. The thick ink left in the cartridge will often flow better through the nozzles once heated.