Basically, overheating your system again and re-setting the temp sensor.
Steps
1. Make sure your Xbox 360 actually has the 3 red lights, also known as the Red Rings of Death (RROD)
2. Get 3 common towels (obviously not paper towels, you'll want towels like the ones you use to dry yourself off after a shower)
3. Remove everything from the 360, including hard drives, wired controllers, games, etc, EXCEPT for the power supply.
4. Wrap your Xbox 360 with the 3 towels.
5. While wrapped, plug your 360's power supply into a wall socket and turn it on. You'll get your usual RROD. Let it run for 15-20 minutes.
6. Afterwards, turn the Xbox 360 off and let it cool for 10 minutes
7. Plug everything back in and turn it on again. It should work.
Tips
* When heating your 360, keep a close eye on it, just in case it might overheat too badly.
* A fire extinguisher would be nice if you feel the need you might want to use it. (Make sure it's for electrical fires)