If you're not on PayPal, it's time! This is the easiest (and FREE) way to accept payments. Yes, you'll pay a transaction fee, but it's unavoidable unless you have people mail checks to your house, and well, that's not optimal.
So, log into PayPal and go to your Business Settings. You'll be able to create an invoice template and some things you want to include on it are:
Logo
Address
Phone
Website
Information about late payment