20 Ways to Use Google Analytics Custom Variables

Track The Visitor ID
Google Analytics doesn’t let you put in personally identifiable information, but it’s generally held that random visitor id’s that you can then compare with your own database outside of GA, are ok. so this way you can track the same visitor even when they’re not logged into your site.

_gaq.push([‘_setCustomVar’,1,’UserID’,’1234567890′,1]);