Take action early
It seems obvious, but the biggest problem that parents and students have is procrastination, especially when it comes to really preparing for college. It's not enough to wait until a week before your child departs, and start packing up all the stuff they will take with them to school. The most important thing you can do for them is to talk about college life, your expectations, what they might expect, peer pressure, money pressures, sales pitches, etc. If they can learn what to expect, and the consequences of poor decisions, you and your child will be much better prepared.
Remember, once they are gone and have made the mistakes, it's too late to prevent them. Besides, they are more likely to listen to you and your advice when college is still a dream and you haven't yet paid the bills. If necessary, you might even think about putting any agreements in writing, much like a contract, so that both of you know what's expected and what's reasonable behavior. If you wait until they are gone, you share in the blame, so talk to them now!