There are always interesting problems that arise. There are always new challenges. New problems come up, and you have to think of either old ways or more interesting, innovative ways to approach the problem. To agree on a protocol and try to keeps things as close to the protocol as possible. No matter how hard you try, people are people and they don't necessarily follow the rules for one reason or another.
Education is key.
The more quantitative you can be in your undergraduate education in terms of math and computer science, the better off you'll be. You don't absolutely have to have an advanced degree, but if you do, you're much more marketable. The more you can get in any kind of project when you're in school, the better. Volunteer for something. Check clinical trials Web sites, medical journals and major medical centers.