Here's a good article by Matt Keenan of Shook, Hardy & Bacon in Kansas City: "Preparing a Witness for a Successful Deposition"--
One way of lowering their level of anxiety is to give them mileposts to follow as they prepare. Something that’s easy to remember but useful. This article shares with counsel my system for witness preparation, with tips and tricks for a successful undertaking.
My approach is based on a simple acronym – PLEASE.
It goes like this.
P is for prepare; L is for listen; E is for exercise control; A is for accept the obvious; S is for stay in your area; and E is for emotion is acceptable.
To get the details, read the entire article. As I can attest from my years of working against Keenan in mass-tort cases, he knows what he's talking about.