My experience was like night and day when I started using an quoting/invoicing plugin called Sliced Invoices in my website. It allows you to add a paypal button right in the invoice that you email to them. It give your clients a convenient quick way to pay via credit card or paypal on the spot. My clients started paying right away as soon as i added the button to the invoice email. Each email you send would have a stamped record of your attempted to collect. Plus it keeps a good history of what you charged for a service to reference to in the future. As an alternative you could you paypals invoicing. I have not uses this paypal feature but I would bet that they have a paypal button on their invoices also.
Send an email a week before the payment deadline.