Seth Godin brings us this post: Four Big Words.
As he points out, over delivering is hard. Really hard. In fact, it can be hard just to deliver consistently.
The key is developing your system for delivering. Run the system over and over again. Refine the system. Improve your results over time and develop good habits. Then push farther.
But not everything is going to go smoothly. So make sure that you are engaging your clients in conversation about your services. Clients are much more understanding of your mistakes when you are openly engaged.
When you try to hide your imperfections, your clients can smell it. And then everything starts to smell bad to them, even if it is fine.
Trust is very fragile. But if you can make good on your promise and then go farther, your clients will reward you with powerful testimonials.