Accounts Receivables and sales operate much the same as Accounts Payables and your purchases. Under the accrual method, you will be entering sales as you earn them and invoice your customers.
Have a simple chart that shows who owes you money each month with a beginning balance which equals last month's ending balance. Enter any payments they've made or new charges they've incurred, and come up with an ending balance. That must equal the amount you list as accounts receivable on your books. If it doesn't, you will need to figure out where the discrepancy is.