Earning to Give, choosing a well-paying career so you have more to donate to the best charities, is a strong contender for being the most effective way of making the world better. [1] It allows you to have a clear positive impact and avoids the replaceability problems of trying to do good work directly. It's good enough that I consider it the standard for effective altruism, the simple approach that all others need to measure up to.

So it's a lower bound, but the question is, how good a lower bound is it? If you're willing to spend your life in whatever way does the most good [2] do you work and donate or do something else?

Update 2013-06-09: more discussion

[1] If you have relatively high earnings capacity on a global scale.

[2] Which I'm not.