I cannot count the number of students I've met who have a story to tell about their GPA. While it's possible to overcome a low GPA and get into medical school, it's even better to avoid the low GPA entirely. Medical schools have more than enough qualified applicants who don't have dips in their GPA, so avoid the following three things, which can ruin your GPA.
1) Take more than the normal credit load at your school (more than 4 courses each quarter/semester)
Your most basic job for getting into medical school is to get A's (and only A's, to the extent that you can). Why would you needlessly make that job harder by taking more than the normal course load at your college? It's true that med schools want to see that you're able to succeed in a challenging course load, but if you're taking a full course load and being involved outside of the classroom, then that should suffice for proving your ability to handle challenges.
If you're sporting a 4.0 through your first two years of college and you want to up your game a little by taking another course, then by all means do so. But if you haven't gotten straight A's, then taking more than 4 classes at a time is dumb. Even if it doesn't ruin your GPA, it's a great way to guarantee that your GPA won't be as competitive as it could be, and that hurts your chances of getting into med school.
Don't forget that you should not study so much that you won't be able to be involved outside of the classroom. Each additional course you take has an opportunity cost (what you could otherwise be doing with that time), so before you load up, consider what you might be missing out on.
2) Take more than 2 science classes at a time
The average student applying to med school has a 3.54 overall GPA, but there's a significant difference between the science GPA (3.44 average) and non-science GPA (3.66 average). In other words, this GPA difference shows what you probably already knew: your science classes are harder.
Whether it's because these courses are curved or the labs take up large swaths of your time, science classes should be carefully spaced to avoid hurting your GPA. A good rule of thumb: take no more than two science courses each quarter or semester. This is most important during the weed-out basic sciences, such as Orgo, but it's still valid for upper division courses too.
If you're trying to find a way to get all of the science courses in, you may want to take a particularly hard course or two at a community college over a summer. As long as your grades are good and you don't take too many (say, more than 3-4 courses total), med schools are fine with this strategy.
3) Double major (or major in something needlessly hard).
Most colleges offer a variety of biology-related majors: biochemistry, neurobiology, bioengineering, and plain old biology, just to name a few. Usually, there's an established hierarchy of which majors are the hardest. The pre-meds at the college know which majors are harder than the others, and often they think that by choosing the harder major, medical schools will give them bonus points on their GPA when they apply.
Here's the thing: most med schools have no idea which major is harder. And they rarely give bonus points for majoring in something hard. The reason is simple: medical schools are ranked by the US News on the average GPA of their incoming class and not by how hard those majors are. The medical schools, then, are looking for good grades, and they won't modify your GPA just because you majored in biochemistry as opposed to general biology.
Not only does a hard major not help you much, but a double major doesn't help you much either. For one, lots of people applying to med school double major. Plus, if you're looking for "bang for your buck," a double major is a poor time investment. Most majors require 10-12 classes. Each class is roughly 4 hours per week (in class), another 8 hours per week of studying, for ~15 weeks total. That's 180 (12*15) hours for each class, or about 1800 hours to get that double major. Think about how much volunteering, research, or other extracurriculars you could do with that time. Spending that much time on such a marginal admissions advantage just isn't economical.
So what should you do to help preserve your GPA?
1) Get A's. Simple, but true.
2) Don't try to be a beast. Just take a normal course load and let the quality of your grades and the quality of your experiences do the work.
3) Under-schedule yourself. You will get sick. You'll have rough weeks. You can't control what will happen to you; all you can do is deliberately leave room in your schedule to anticipate these setbacks. That means not taking too many classes and getting overly involved in extra-curriculars.