Sometimes there's nothing you can do...
Take Reddit for example, 95% bounce rate, nothing will keep their interest.
Take Google, people searching for stupid long tail phrases w/ no relation to your site, just similar keywords, so they bounce at 90%.
Take Forums, 50% bounce rate, people want to get back to the forum fast.
Bounce rate today is just so skewed, people are flaky in general and don't even know what this is..
You could add better content, re-design your site, etc. But certain people will still bounce.