Kevin Potts has a short and sweet article about Better Invoices in the new ALA. Highlights of the doc format:

• The word Invoice
• Tax ID #
• All names and addresses
• Date the invoice was sent
• List of services with dates
• Terms (he has a good example showing carrots and sticks for timing, to which I would add dates to be more clear)
• Mailing is better than emailing
• Include a thank you note
• Send within 48 hours after, never before, the milestone
• Don’t time it to arrive on a Friday
• Accounts Payable needs a nice, big number at the bottom

There’s some good tips in the comments too…’If you have become ‘friends’ (I use the word very loosely) with your clients they will feel obligated to pay you on time. Try joking with them or getting just a little personal. I find sending links to pictures of a recent addition to my home helps out greatly.‘