I send invoices at the end of the month and payment is due in 7 days. Never had anyone not pay on time. If I did work would stop until paid in full. I’m not a bank…
I keep an Excel spreadsheet for the Overall Costs of the Job on my desktop computer. It has columns for Labor, Parts, Materials and a Description of the work done, plus a column for Payments Received and Invoices sent. When the job is complete I have the total hours spent, materials used, etc.
On my laptop in the garage, I keep a spreadsheet for Time and Materials that is updated regularly throughout the day.
If I get a materials deposit, I keep a separate sheet, where each material order is entered with the date and supplier. This gives me an accurate running balance and it's easy to access. All receipts are kept in a folder in the file cabinet.
At the end of the month, I create an invoice from data retrieved from the Spreadsheets. I bill monthly with the stipulation that the owner can take his car home any time he wants, as long as he is paid up.