One of the most common questions that I was asked on my speaking tour was, "What if I accept an internship but then get another that I'd rather take?" This happens all of the time and it definitely happened to me during my college years. Here are my 3 pointers for that situation:
1. Assuming you actually want all of the internships you applied for, just say yes when you get an offer, unless you've already accepted something else that you'd like more. If this happens, thank the employer for the opportunity and tell them you've already accepted another opportunity but you'd love to stay in touch. DO NOT ignore the employer's emails or let phone calls go unreturned under any circumstances.
2. If you've already said yes to someone and another opportunity comes along, go ahead an accept it. And then its time to deal with the previous employer. CALL them. Don't email them. Phone calls are more personable. Be honest and tell the employer that another opportunity came along and apologize for inconveniencing them in any way. Don't refer to the new opportunity as a "better" opportunity, just another opportunity that better suits your career goals. Thank them for their time and say that you hope to stay in touch.
3. Remember, its always about keeping up the relationships, even with the internships that you do not accept.