Spider
Use examkrackers, in my opinion they have the best material out there. I've studied from kaplan's comprehensive book and also Flowers, and I can honestly say that for the bio, phys and the verbal sections EK is much better than the other two. EK's chemistry section leaves a little to be desired, though its still pretty good. The thing i really like about EK is that they teach not only the concepts but also how to "work" the exam.
Plus, Ek has the 1001 question series for the physics and chem, which I have found very helpful for identifying weak areas in my knowledge base. I find that sometimes I think I have a firm grasp on a concept, then when i go and start answering questions from the 1001 book, I find that my grasp was not as good as i had thought. Usually after answering many questions from the book I have a very strong understanding of the concepts.
How were your nerves during the exam? Excessive nervousness can destroy anyone's chance of doing well. If you were too nervous, then you need to gain control over your emotional state. There are numerous ways to do this, hypnosis, relaxation techniques, herbals (kava kava), beta-blockers etc... I'm not recommending that you use drugs or herbals (the reasons for which are many and better left to another discussion thread), but some people do resort to them. Regardless, you do need to get control over your state, as nerves can make even the most prepared individual fall apart come test time. You would be wise to figure out how to get control over the situation if that was the problem.
Whatever you do decide on, just make sure you do not prepare the same way you did last time! If you are not studying properly, then simply putting in more time is not going to help you. So change your materials, change your technique, whatever, just change something.
Anyway, sorry to hear about your bad test experience, I feel your pain. Now you have to look at this as an opportunity, its a chance for you to show the adcoms how much you can increase you mcat score by. Going from a 20 to a 30 would be pretty damn impressive, so go do it!