A simple email to your contact in the Admissions Office is sufficient. Be gracious and brief. Thank them for the opportunity to consider their school, and you enjoyed meeting them and their students. After considering all of your options, you've made the decision to attend another school, and are declining the offer of a seat in their program.
It's good form to let the school know as soon as you've made your decision, so that the seat can be released to another candidate, most likely someone on the wait list.
Congratulations on making your choice! Now the real work begins!
15 Years as Director of MBA Admissions - Now Working For the Applicants as the MBA Admissions Coach
Wendy Flynn is the MBA Admissions Coach, providing comprehensive MBA admissions consulting services. As the former Director of MBA Admissions for 15 years at a top-30 MBA pgoram, Wendy holds deep expertise in admissions issues for Full-Time, Executive and Professional (Part-Time) MBA Programs. Through her blog, MBA Expert Insights, she brings the view of the Director of MBA admissions to MBA applicants and others.
MBA Expert Insights Blog: http://www.mbaadmissionscoach.com/blog/
Member of the AIGAC, Association of International Graduate Admissions Counselors