Over-Confidence is the Mortal Enemy of a Salesperson

"Don't worry, I got this."  "Don't worry, this customer is in my pocket." "Sam wouldn't do anything until he checks with me."
I cringe when I hear those words from a salesperson.  Your customer has one purpose in mind--if he wants to keep his job--to make the best deal for his company.  So, while you're out patting yourself on the back because you feel that your customer is in your back pocket, think again.
The cardinal rule of a good salesperson is NEVER to think he has the order in his pocket; NEVER to think the customer will call before he decides to go with the competition.
Fear of losing the order is a great trait to have as a salesperson.  As long as you fear that you will lose the order, you will stay with it until the check clears.
One thing that customers don't like very much is that constant call asking if they placed the order yet.  So there's a thin line between being a pest and being over-confident.
You need to call, but you need to be offering something new when you call.  You need to develop a follow up strategy (just like a post sale strategy) that keeps you communicating with the customer so that your voice is the one that rings in his head (in a good way) when it's time to place the order.
For example, lead time update is always a good reason to call; any product updates is also a good excuse.  The point is, have a reason to call both before and after the sale.
Never assume the order.