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.
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.