How to decline a job offer?

I applied for a job and received an invitation for an interview. I accepted. After looking into it, I did not think this was the right fit and did not want to cancel the interview. I did the interview and nailed it and they offered me the job. How do I respectfully decline the offer?

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official