Using invoicing systems to make it simple - While you can make up your own invoice or use a free template, it's often easier to use an invoicing system that offers more in depth features. Some invoice systems, like Harvest, offer the option to create estimates for big projects, keep track of multiple clients and send reminder emails when payments are late. Another option is Freshbooks, which offers similar features but includes profit and loss sheets, as well as trackers to help you manage your time.
Accurate billing is extremely important when it comes to independent contractors and small business owners. You can't afford to lose a customer over something that should be done right the first time. Business owners who provide detailed, prompt invoices straight to the customer's inbox come out way ahead in the credibility stakes.