This is pretty straightforward, I think. I just update the INVOICE NO. and DATE as well as the other fields:
"To" is the person who receives the invoice. "Ordered By" is the art director or other person I've been working with on the actual job. Sometimes they are the same person, but if they aren't, it's good to include my contact's name in case anyone in the billing department wonders who the hell I am.
As much info as I can include about the specifics of the job. If the client has an official description or purchase order number, that goes here as well.
Usually "none," but occasionally I have to charge for excessive revisions, kill fees, and so on.
The most important part! I write this number in a bigger font size than anything else on the page, so it's easy to see. I have that thing about "Due within 30 days" on there to protect myself, but I'm not a big stickler about it. I've been lucky enough to have pretty good clients but if I ever have to go after a deadbeat this will come in handy.
My contact info and Tax ID stay the same, so they're a permanent part of the template (background) layer in Photoshop. (I used to have a crappy old Word document, but it was ugly, featured design elements from an outdated version of my site, and probably didn't even look the same on all my clients' computers.) Once everything is updated, I save the file first as a .PSD and then a flattened .PDF, which I email to the client.
A note about color: When designing this invoice, I tried to consider the needs of the clients who would be using it. Since I send it electronically, I was tempted to go full-color, with a bright background and all sorts of other details, but for all I know, some of my clients may need to print off hard copies and would resent the drain on their printer ink. Instead, I settled on the more conservative version above, which should work well even if printed out in grayscale.