Generally if you are turning down a job offer you need to leave a polite voicemail message or send a short note thanking them for their time and explaining things.
Thank them for the offer in this difficult economy. However, due to personal circumstances or an alternative job offer made, you must respectfully decline.
About the only way is to be truthful. It's best to talk to the person in person. Tell them you have a better offer or whatever. There is a chance they could find out the reason on their own.
