Is the only way to lose one's citizenship if born on American soil is denouncing it?
[bit.ly]
I may be hazy on all the options, but I know renouncing US citizenship as part of the process of establishing citizenship in another country is one way, and serving in the armed forces of another country can lead to loss of citizenship. (Another involves being an active, professional pirate in international zones, but this is hard these days, believe me. Real hard.)