Full-time positions are tough to come by; internships are even more difficult by virtue of the number of positions available (generally a couple at each firm). Still - some students will be in the fortunate position of holding multiple internship offers. For those of you, congratulations! Here are a few critical points to keep in mind throughout the process:
1. Don't take forever to answer.
This one is very important - the longer you take to answer a company, the less excited they think you are about joining. Don't keep a firm waiting for too long - a week or two is understandable, especially if they know you have other offers, but don't take longer than that, and give them a valid reason. At some point, you will have learned everything there is to know about the company - you don't need details about which cubicle you'll sit in, what type of laptop you'll get, and what the dress code is on Fridays.
You've built good will by receiving an offer. Learn what you need to know and then make a decision. I've heard stories about students who received internship offers but took so long to reply that they lost all partner-level support - that hurt their reputation and prospects when they did accept the offer. Don't be that guy (or girl)!