Whatever the case, make sure to invoice your clients consistently. You're more likely to get paid on time when your client receives invoices regularly on the first day of each month, than getting them sporadically.
Notes: Many people overlook it, but the notes section on the invoice can be invaluable. A simple phrase like "Thank you for your business" will leave a good impression to the customer and increase the chance of an invoice getting paid faster. You can also include Payment Instructions, Warranty Information or even a personalized message in this section to make them feel really special.