I had never written an invoice before three years ago when a magazine told me to send one to them. I was like, "Huh?" (I tend to always be like that -- I should learn to nod sagely.) So, since I knew that Microsoft Word had a huge template library, filled with resumes, flyers and even menus, I looked and found they had one that fit my needs.
The template library also has copies of letters asking for late payment and threatening an assessed fee, which also could be handy.
Anyway, after I send in my submission, I usually send my invoice with a carefully selected stamp and usually a post-it saying, "Thanks!" or "It was great working with you!" Anything that gives it a little personalization.
As I said, sending invoices are my moments of zen.