In the beginning of the Avengers movie, Black Widow finds Banner in India, treating sick children. This isn't just a humanitarian endeavor for him; Banner purposefully surrounds himself with injustice - with poverty, with senseless death - so that he could be constantly *angry* at something. 

Being constantly angry allows him to keep his anger under control - it's not a sudden spike of anger that disrupts his concentration and lets the Hulk out, it's a constant, background anger that lets him decide when to unleash the green beast. 

From the script:

<pre>
                       NATASHA 

        You know, for a man who's supposed to be
        avoiding stress, you picked a hell of a place
        to settle.

                       BANNER 

        Avoiding stress isn't the secret.
</pre>

This doesn't say it explicitly, but implies to me that his choice of location and activity are part of his secret.