I recently took a pre-employment poly, the second one I've taken in my life, and I am compelled to say a few things. 

For those looking around the internet to find helpfrul information on the polygraph; what it is, how it works, how to pass, etc, and who have stumbled by chance onto this site as I have, read the book. 

The Ebook offered on this site for free is all you need. 

I will say also that the single most important thing I took away from it was ease of mind.  If you start out as I did, ignorant of how the polygraph works and how a polygrapher operates, you will find that much of what is written in the book alleviates the intimidation you may feel and removes the mystery.  This will allow you to relax during the exam much more easily, which is the most important thing for a successful outcome. 

Read the book through a couple times.  It helps.

Save the thirty dollars you could spend on Doug Williams' book and buy yourself a celebratory dinner after you pass with the techniques and information found here for free.