For soon-to-be college students, here's my five mistakes:
1) Screwing off and getting mediocre grades as a freshman. They will stick with you and will make you regret having assumed that you could put in high school-levels of studying and still get an A. Chances are, by the time you're a senior, you'll know what's expected and can coast to good grades, but as a freshman, don't assume this to be true, because you'll still have to adjust even if you took 12 AP classes.
2) Not settling on a major immediately. This goes contrary to popular wisdom, but the sooner you decide and stick with it, the better. You can get your requirements done sooner and leave the fun electives to your junior or senior year. Also, if you get the requirements done quickly, you can take much smaller classes or graduate level courses in your major where profs will get to know you well and serve as great references.
3) Not drinking before college. This goes along with number one, but I drank about twice before college, mostly because I had a lot of Mormon friends. Suck it up and get drunk with Dad some night over the summer--the more you get used to that feeling, the smarter you'll be with alcohol. I laugh at this now, but four years ago I wasn't alcohol-adverse, I was just completely inexperienced and figured "I'll do it in college."
4) Should have gone to the best school I got into, or a good public school. I just got a great feeling at my school when I visited there, but seeing people who attended worse schools, have less debt, and have similar jobs to me kinda sucks. Either shoot for the top, or go to a good public school and work your ass off to distinguish yourself.
5) Interned. I worked for my state governor's policy office, which sucked major balls, before my senior year while my friends were at financial and consulting firms. When I applied for jobs this fall (after deciding to let law school wait a year), interviewers weren't particularly impressed or convinced I was interested in that career path. All I got out of the dumb summer was the knowledge that I don't ever want to work in government.