It is VERY difficult to become an un-citizen of the US. The only way to become an "un-citizen" it to go to a US embassy, hand them you passport and denounce your citizenship.
Other things that MIGHT make you lose your citizenship would be:
-Becoming an officer in a foreign military
-Serving in a high-ranking political posistion
It is very hard. Look at John Walker Lindh, he served with the Taliban and still is considered a US citizen.