Getting down to the business of payment, in most cases, runs smoothly. Unfortunately in some cases, just because you send an invoice, it doesn't mean you are guaranteed a payment. The best thing to do is agree payment terms from the offset, so customers know exactly when payments are due from date of the invoice.
Another thing you need to look at beforehand, is how to send the customer the invoice. The lack of payment could just be because they have not received it, so you need to agree before you provide any goods or services whether you are going to post, fax, or email the invoice. If you are going to email, you need to think about if you want the client to sign the invoice or not. Although this step isn't necessary, it cannot be done electronically so the option would not be viable.
Convince yourself of the “Invoice creator” quality based on our sample invoices: