Chris Coyier recently published a decision tree on his site, CSS Tricks, that demonstrates a plethora of ways to horizontally or vertically center things in CSS. As standalone solutions they all have their caveats, but combined, they're an unstoppable centering tool that works cross-browser in IE8 and up consistently.
Source files for the Tuts+ tutorial: The Holy Grail of CSS Centering
Available on Tuts+ September, 2014