1. It Pays To Do Your Homework
For bigger customers you may need a signed contract and a purchase order. Find out what you need and their payment terms in advance. For example they may have money left in a budget and need invoicing straight away. For smaller customers make sure you agree the price in advance so there is no query on your invoice. Obtain the mobile number of your contact at the company and if possible find out the name of someone in the credit control department in case of a query.