How to turn down a job after you've already accepted?

I was presented with a "Contingent" offer of employment to go to work for a company. If I take the job, it will require that my family and I would have to move. This company came to me out as I am already employed and enjoy the job that I have now. They are offering a salary that double if not almost triples what I am currently making. I've known this company for many years as I dealt with them as a customer, and honestly, I did not like the company from a customer's standpoint very much, but now to go work for them? Anyhow, they presented the offer to me almost three weeks ago and pressured me to make a fast decision. So due to the pressure, I agreed to take the job and signed on the dotted line. My potential start date is in 5 days and if I go, I will have to leave my family behind for approx. 6 months. For the past week and a half, I have been debating as to whether or not I sincerely want to take the position and have come to a final decision that I do not want to take it and would like to stay where I am at. I know that when I contact them to decline the offer, they are not going to be very happy as they are needing to fill this position very fast. The company made a bad decision as they failed to seek out any other possible candidates for this position, so they will have to start from scratch once again. How can I turn down the offer with dignity?