Without letting them know that I was just using them as interview-practice?
(It was a great interview, too - they asked all the hard, touchy-feely questions - I feel primed for real interviews now after going to this one where I had no intention of taking the job.)
ps - All’s fair in love, war, and employment.
Say something along the lines of “Upon further reflection, I don’t think this would be a good fit at this time.”
Tell them you took another position just before hearing from them, but thanks for the time and consideration.
Tell them your personal situation has changed and you won’t be in the job market for an indefinite time to come.
Tell them the truth, in which case you probably won’t be hired by them ever in the future, but won’t compromise your integrity.
Take the job.
Thank them sincerely for the offer (by phone conversation, not email or letter), and say that you’ve chosen to pursue another opportunity.
“This is featherlou, and I wanted to call because I’ve received your offer letter (or picked up your message, whatever). Thank you so much for the offer. I really appreciate your taking the time to talk with me. I wanted to let you know in person that I’ve decided to pursue another opportunity. Thank you very much for your consideration for the position, and I know you’ll find the right candidate.”
Short and sweet.
Ask them to double the salary they offered. If they agree, tell them you’ll also need a personal masseuse. If they agree to that, take the job. 
So, just not returning their calls is out? 
(Not like they don’t do that to us all the time!)
I like the way you think, Giraffe. Maybe I’ll try that. What have I got to lose?
Simply say that you’ve decided to go elsewhere. This is true, even if you don’t have another offer yet.
Insult their mothers.
Their mother’s what?
Figure out how much money you would have to get in order to make such a job appealing.
Ask for that.
3A. Get the job, or
3B. Don’t get the job.
Nothing beats the sound of hands slapping your bare, hairy ass!
Oh, why bother?
Napalm. Lots and lots of Napalm.
Well, I called them and told them I was going in a different direction; they sounded very disappointed. I don’t think I’ll do this again; I feel a little guilty for misleading them, no matter how badly employers have treated me in the past.