A way to seperate billed hours from non-billed hours

A way to seperate billed hours from non-billed hours

Perhaps there is a way to do this already..  I often bill hours here and there, sometimes a side project will have a few hours here and there that I only bill at the end of a month or quarter.  I need to be able to separate invoiced hours from non-invoiced hours when I create invoice or at least on the timesheet report.   I suppose I could make a note on a task when I've billed for it..  but thats not very elegant.. and I know I'll forget.

Any suggestions?