If you have been involved in sales or customer service for some time, then you have most likely heard the phrase "under promise & over deliver". Simply put, this means that when setting expectations, it is best to do so in a manner which enables you to easily exceed them. For example, if you are planning to meet with a customer and they ask how long it will take for you to arrive at the location, overestimate the time. So if you know that it will take you 30 minutes to get there, tell them you should be there in about an hour. That way when you show up early, you set the tone of being punctual.
Many of us get ourselves into trouble by trying to be too precise with our promises. The challenge in this approach is that we set our expectations too close to what the actual results will be, therefore not leaving any room for unforeseen circumstances that arise. We all have distractions that can get in our way, and under promising & over delivering is a fool-proof method of dealing with those distractions.
Let's assume that a customer (or coworker or spouse or whomever) asks you to check on something and get back to them later with the results. You know that it will take you approximately one (1) hour to get the information. In this instance ask them if it would be okay if you got back to them in a few hours or even the next day. So when you call them back in one (1) hour, they will be impressed with your timeliness and you will also set a precedence of being accountable.
Too many times we subconsciously want to impress the other person so bad that we set unreal expectations for ourselves. If you tell the person in the above example that you will get back to them in about 20 minutes, and then take one (1) hour, you have set a precedence of being late as well as sending the message that you do not value the other person's time.
In addition to showing that you can be counted on, this technique also prevents you from being rushed when those unforeseen circumstances DO arise. So do both yourself and the other party a huge service and UNDER PROMISE & OVER DELIVER every chance you get!