I went all out for OCI, did everything I could to prepare, and it paid off in spades. You CAN prepare for this.
1. No matter what, grades and school rank are 75% of the battle. These two factors are going to determine your success far more then anything else. So go to a good school, and get good grades. Everything below will only have so much of an impact.
2. Get as many interviews as you can. Don't listen to people who say otherwise. You never know what firms you might like, and regardless, the practice is good. And ITE, cast as wide a net as you can. During OCI, I interviewed with literally any firm I could talk too, even outside of my primary target areas. I had around 40.
3. Prepare for interviews. I went straight from UG, and had little interview experience, so I was not sure what to expect. Search the forums here. Make a list of every possible question you think you might get asked. I had about 5 pages of questions. The most common include:
1. why this city?
2. why this firm?
3. Tell me about your summer?
4. What was the most challenging/rewarding thing you did this summer?
5. Would you rather do corporate or litigation?
6. Tell me about X (something interesting on your resume)
Those questions are about 75%+ of any interview. Other then that, be prepared for curve balls, and also to talk about EVERYTHING on your resume. After 5-10 interviews, you will find that you fall into a rhythm where interviews become easy. It's just you repeating the same stories and anecdotes again and again. Also remember: be energetic, be enthusiastic, and be engaged. This can be hard as the day goes on, but constantly remind yourself of this.
Also - it helps a lot of you show that you are certain you wan't to do something - if you want to do corporate only, say this. They are looking to decide people between litigation and transactional work, and if you are decisive about it, it shows you've thought about it and they can better meet their hiring needs.
4. Be prepared to ask THEM questions - this is especially true in the callback stage. Print out the chambers/vault/ATL guides for each firm and the particular OFFICE and read through it in the 10 minutes before the interview. Be prepared to ask some questions. My favorites were:
1. What differentiates you from other peer firms/whats your favorite thing about working there?
2. Do associates have a lot of control over their career development and what they can work on?
3. Do you work a lot with the other offices in the firm?
4. What about X (practice area)?
5. Do callbacks ASAP. Like others have said, you will get better at these over time. Realize they are NOT like OCI interviews. I found that callbacks were rarely interviews - most of the time, your interviews are "meetings" - they will talk to you, and you are expected to find out everything you can about the firm - ask them about their day to day, practice areas, opportunities, associate life, if things are busy, etc. They are trying to get to know you, and you are expected to find out about the firm. They will NOT ask you questions, or at least, the will do so at a much reduced level. You have to show them you're a cool person who they would want to work with.