Well, the obvious response is to become either a scientist or an influential philosopher/large-scale ethical role model. The problem with either, however, is also obvious: what if your effort doesn't achieve anything? What if you spend years working on a cure for some social or medical ill and someone beats you to it by a week? Or worse -- what if you never get anywhere, whether that means finding no cure or influencing no-one to act more morally?

Thus it would seem that to be a truly effective utilitarian, you'd need to devote some time to sure-fire utility, such as becoming a doctor (in becoming a doctor you're pretty much certain to help at least SOMEONE in some rather large way, for instance) and use that experience of creating utility and making decisions for maximum utility in your everyday life to reach and teach others how to act morally.

I am a utilitarian myself (an admittedly fairly hypocritical one). I choose to devote my life to the creation of art, which I believe is nothing if not a utilitarian pursuit.

Excellent, excellent thread idea by the way. Really has me thinking and I'll be following this thread closely.