Anyway, sending out invoices is my moment of zen, as Jon Stewart would say. I get to tally up what I'm owed and send it off. I carefully choose the stamp -- maybe creative but professional (I thought superheroes was a little too casual, so I opted for the boring Liberty Bell) and send it off for payment. You can e-mail them, but there's something about the actual mailing that seems to make it for me.
All my freelance money is going straight to my grad school fund. So far, with that money and some savings, this year is paid for! It's always a great day when you don't go into debt for your education!