Be very honest with yourself when evaluating your interviewing skills and techniques. Instead of blaming the firm for overhiring, try to understand what you may have done or not done during the interview to prevent the firm from hiring you. One method of preparation that seems to work quite well is to sit in front of a mirror and conduct an interview with yourself. At first you will feel very stupid, but after a few minutes you will get into the rhythm. You will be able to evaluate your body language and the way that you are responding to questions and delivering your answers.
Some candidates even videotape a mock interview.
My other suggestion and the one that I think is the most valuable is to have a friend interview you and then critique your interviewing skills afterward. I know that you are starting to feel desperate but please hang in there. Make sure you are getting in touch with every possible contact and are networking at every opportunity. Get yourself out there everywhere and network like crazy. Just about every day and night of the week there are bar association events where you should be trying to meet potential employers.