Get your hands on as many legitimate practice tests as possible. You should take each test under the same conditions you would the actual MCAT. This includes time restrictions, environment, desk space, etc. It's been a few years since I took the MCAT. That was back in the day when the tests were on paper (gasp!). Being able to simulate the test conditions and to build up the stamina required to take the test will pay dividends.
However, if you're averaging a 26 under non-testing conditions, you need to slow down and try to find out what's getting ya. The foundation of the MCAT is knowing a bunch of stuff that allows you to speak the same language as the test. The test is entirely analysis and problem-solving. You need to find out what the disparity is between what you know and how you approach each problem.