Promise on what you know you can deliver.
If you don’t deliver on your promise, your customer will be upset – wouldn’t you be? You want to manage expectations upfront and do everything you can to deliver. Even over-deliver if you are able.
One of the things I loved most when I first started using Amazon.com’s free shipping is that while they projected delivery within 5-7 days, packages often arrived early!
Follow-up with the customer on the status of the deliverable.
Let’s face it, things happen, unexpected problems occur, and sometimes you have no more options to try and get the work done on time. Let the customer know before your deadline that things are running behind schedule and specify how much more time you need – your deliverable may affect other plans and projects.