I use QuickBooks (duh, eh? see'ins how I do books for a living?

) to do my invoicing and tracking (all 2 clients

). I print the invoices to .pdf format and email them to my clients monthly on the 1st w/a net 15. I run an A/R report prior to invoicing to verify that there aren't any outstanding invoices.
I put my payment terms/penalties, etc. in the original consulting agreement and put NET 15 on the invoices. Obviously, no problems with payments yet!