Thanks for a great article. Nuts and bolts stuff like this merits almost as much attention as high-design technique. (It, too, is all about the usability.)
As long as we’re sharing, here are a couple of things I do that make freelancing a little easier:
To achieve that wholesome branding goodness, I design the invoice in a page-layout program (I use InDesign), trying to be careful not to “overdesign.” Then for the guts I place an Excel spreadsheet, so that Excel does all the number-crunching automatically as I enter blocks of time on the job.
Also, I’ve adopted a convention of making the invoice number correlate semantically to the date the work was begun (e.g., TM-040711-XYZ, where TM are my initials and XYZ is an acronym for the client). Then as you suggest, date the invoice the day it is sent.
This method provides two advantages:
1. It indicates the overall duration of the billing cycle (the difference between the invoice number and the invoice date), and
2. The invoices order themselves both chronologically and by name in your folder.
Obviously not as sophisticated as say, iWork or Basecamp, but good enough for us small fry.