[entropyneur]

The best way to do it is probably to track clicks on outbound links using javascript.

[stevendaniels]

Aren't there a few cases when this method won't work?

[entropyneur]

Are you referring to the fact that the browser will interrupt your tracking request because it already started loading the linked page? I haven't really tried, but I believe this can be dealt with if your server-side code expects it to happen.