One of the most important things that I personally learned in both client relationship building, and selling is determining when to "hold your tongue". Too often we want to communicate our personal agenda, knowledge, and advice only to discover it is not desired, needed or even necessary. Knowing your audience is critical to understanding when they will really listen to you and understand the point you are delivering. This becomes critical in both presentations and even one on one discussion.
It is so tempting to want to redirect a conversation to something YOU want to say (or hear yourself saying) that has little to do with the objective of your conversation. Being able to step back, hold your tongue (and even the thoughts) is a discipline that can be the difference between a yes and a no.