I need a good 12V power supply from time to time. There are a lot of good laptop power supplies around that can give out about 5A. Is there a good, proven recipe out there that can do the voltage conversion from 19V to 12V? I don't want to use a resistor-based voltage divider, or a string of 10 power diodes.
The good and proven recipe is in the laptops. They use buck regulators everywhere on the motherboard. So I would recommend it to you as well. Just use buck regulator with parameters you like.
A linear voltage regulator (as suggested by Heart in the comments) such as the 7812 will do the trick, but for higher efficiency you should consider a switcher. The first that comes to mind it the LM2596 (adjustable output voltage), but you should do some research. There are many alternatives.