Imagine you got a very low score on your MCAT, and think about how you would approach retaking it - how much time would you give yourself to study? 2-3 hours a day? Would you be willing to pay $1500 for a prep class, or $50 for a couple of review books?
The interview is, at least in this case, as important as the MCAT for you, because it's the rate-limiting step for your ability to get into medical school. So I really suggest taking all of the time and money you might have been willing to spend on MCAT review and put it all towards improving your speaking skills. Consider it like a second job, or at least a hobby you work on every day and are willing to spend money for.
I definitely second the idea of Toastmasters and going to every mock job interview you hear about. You might also want to get a copy of
How To Win Friends and Influence People - it sounds cheesy, but it really does help a lot of people make good first impressions.
You should also consider calling the schools you interviewed at this June (while the Admissions Offices are between entering classes and have more time on their hands) and ask if they can give you any suggestions on how to improve. It can't hurt, and they might just save you a lot of time and money by zeroing in on something you hadn't thought about.
Good luck!