I began scoring 23 on my 1st practice MCAT, and was able to do a 29 on the real deal this April.
First, obviously, identify the weak areas. Do you suck at verbal? Home in on that and make it better. Second, for the sciences, make sure you KNOW the material, not just that you think you know it. I knew over 100 formula cold for the Physical Sciences part. That alone raised my practice test from a 23 to a 26. Have someone else - preferrably someone that knows no science - ask you questions on the material that you're studying. Can you explain it to them so that they understand? I realized that I couldn't tell my girlfriend why acceleration was acceleration, and it hit me that I really didn't know it.
There's a spot on the AAMC website, under MCAT, that gives the outline of the things that are fair game for the MCAT...you have to at least know them. And the science courses covered by the MCAT aren't post-grad level...there is some fairly simple concept being tested by even the most obscure question - learn to find that. And do all the practice tests from AAMC...the concepts repeat themselves, in my opinion. You've gotta know waves and harmonics, 'cause I saw those all over the place during practice and - voila - there they were on the MCAT.
Study hard and good luck.
dc