Here is what i do: For every shop I perform, I have a worksheet, cheatsheet, whatever you want to call it. On it, I have pertinent areas of the shop outlined - this could be the visit number, address of the shop, date of the shop or whatever will trigger memory and data acquisition. To this sheet, I attach my hard copies of receipts submitted, business cards, or whatever links the shop to the report.
Most importantly, I have a space that asks for the date of invoice, invoice amount and invoice ID. Like others, I always submit my invoices immediately after submitting my report. So, for instance, if I did 3 shops that day, I file the reports and then, file the invoice as one for all three. After submitting the invoice, I write down the invoice ID and then save the invoice to a folder on my confuser labeled for the year.
Last of all, these sheets are filed into a folder for the specific month and year in which they were performed.
Since doing that, I've never had an invoice not submitted.