3.38, 35S here. Move to the bold part if you want to skip the backstory. BTW, this thread perhaps shouldn't be in this forum but I'll answer anyway.
First, let's dispel some myths - no, I'm not dumb as some people think. No, I didn't party all of the time. No, I'm not simply a good test taker.
For many people, poor grades are the result of something other than laziness or stupidity. In my case, there personal issues weren't really present for the most part either. I attribute my issues to a couple of factors. First, I had essentially no support because I had no idea what I was doing or where to go. I grew up in a rural area, excelled, never studied. Was pretty talented as a musician as well. I got into Johns Hopkins and WUSTL for undergrad but chose not to go because they were too expensive. I went to my 'backup', University of Michigan. Arrogant? Well, I wouldn't say arrogant. Just a guy set up for a textbook example of hubris in Classical literature. All my life, I'd just been told I was smart. All my life, things worked out perfectly.
I didn't do too badly at first, but, my grades slipped with each semester. Each time I tried to fix them, I'd do worse. I had never built study skills in high schools. Plus, I had never had to ask for help in my life. It sounds stupid, but the idea of asking for help never even crossed my mind until late in Junior year. The final straw came when I did poorly in biochem after working harder than I ever had in my life, staying in the library until 4am every single weekday. The problem was that I tried to memorize the whole book instead of focusing on what the professor wanted. Dumb, I know. I still believed that I would be better off by doing things "properly," aka learning far more than was necessary. Coupled with the constant sleep deprivation, I really didn't learn anything properly.
After that, I gave up. Pulled a couple Cs by doing the bare minimum. Found solace in a new relationship and in an a cappella I joined.
Fastforward to the summer between Junior and Senior year. I was miserable because I realized that I just wasn't going to get into medical school (longtime dream). I hadn't taken the MCAT, hadn't shadowed, and had no recommendations. I only had 3 upper level science courses left in my undergrad career and had to get rec letters from all of them to be safe (MD schools want 2 science letters, 1 nonscience - faculty letters very preferred).
That summer, I decided to change. It was hard, to say the least. I quit the a cappella. Studied for the MCAT for two months. Shadowed an MD and a DO. Note that this was happy coincidence - I was intending on shadowing the MD, but he worked with a DO in the same office so I followed both around.
Why do I think I did well on the MCAT? Well, I studied from a prep book (Kaplan). Other than that, I bought 3 practice tests from the AAMC and 3 Kaplan practice tests. I did the Kaplan tests first and put them in a rotation, eventually doing each test twice. Same with the AAMC tests (including the free one). People say that doing a test twice is worthless - bullcrap. Unless you're scoring perfectly on your practice tests, you still have more material to work with. Score as high as you possibly can because most likely, you'll do worse on the real thing. Don't neglect the writing portion. It's not that crucial, but I personally think it looks bad when someone gets a 33 and an abysmal writing grade.
Basically, the practice tests showed me what I should've been doing all along - reinforcing knowledge by thinking critically about it. I had tried brute force memorization most of my college career.
SO, TOO LONG, DIDN'T READ MY SOB STORY? MOVE AHEAD.
My tips:
Set aside at least 2 months to prepare for the MCAT. You don't have to do much each day - even an hour or two of reading / practice is adequate. For the first couple of weeks, focus on reviewing the material. Use the questions in each section of the Kaplan book to test your mastery of the material. Learn the equations. Do this quickly because I think you should spend as much time on the practice exams as possible. Do all of the exams you have. Then, do them again. If you don't have time to do a practice exam, do a section. Be sure to do the entire thing at least twice though. You want to get that "testing center" feel down. Done with everything? Get more material. I borrowed some Princeton Review stuff from a friend who had finished with the MCAT. If you can't afford materials, do this as much as possible. You should be able to find somebody who's willing to lend you stuff for cheap/free.
If you really need it, check out the MCAT forum for a test schedule. Somebody wrote a super-detailed one. Note that I didn't use it, but I think it's worth glancing at.
Test day - basic test rules apply. Sleep well. Get there early. Take a deep breath because the hell is almost over. Treat it like an endurance athletic event - I brought multiple water bottles and a ton of granola bars. At each break, I'd drink and eat a Quaker 90 cal bar to replenish my blood sugar. Use the bathroom. Stretch. Look at yourself in the mirror. This is your time, so go in, be a boss.
One disturbing thing - be wary of your testing site. I heard from someone a long time ago that their proctors didn't allow them breaks. Nobody argued because they didn't know better. As far as I'm concerned, that's totally unacceptable. I know I needed the breaks.
Good luck. The MCAT is a standardized test. In other words, 100% predictable. It's not an undergrad exam where the prof tests you on whatever they like. You have all the time you need to adequately prepare for this. Don't be scared. Own it.