If you're feeding half the voltage does that mean you're using a single diode as a half wave rectifier directly into the 7812? That's going to provide absolutely no regulation at all. The 7812 datasheet I got from National was useless, no example circuits to go from.
Take a look at this circuit real fast **broken link removed**
Instead of the zener/transistor you would use your regulator at that point. The full bridge rectifier and large electrolytic cap are required, a half wave rectifier will have a LOT of ripple when the regulator is loaded.