Re: Keeping Professional Relationship(ties) after turning down the job
^ I concur with Umar here...
In my case I had two offers: One from a small company and the other from one of the "big four", so I clearly explained my decision to the VP of the small company as to why I wanted to pursue the latter (I did not tell them exactly which company I was joining other than the description I have just given), and even referred another person to him whom I thought would be a great fit for that position.
Furthermore, try to avoid sending just an email: Do not hide behind written media. Speak on phone, or better yet, in person so the guy you are turning down knows you have it in you to take a stance and defend it because you really believe in yourself. If the guy you're turning down is a professional, it would go a long way trust me!