Before I came to college, I wish I would have better prepared myself for missing everyone back at home. I had great friends that were like my family, and I didn't mentally ready myself to be away from them for such a long time. Because of this, I didn't put out as much effort to make new friends here at school, since I was so focused on my old friends. If I could go back in time, I would have made sure to make room for new friends while remembering my old ones.
Don't pack so much. You don't need all of it. Learn good time management skills before you get there. There are so many activities on campus and it's hard to stay away from them, but sometimes you have to say NO to them in order to still do well in classes. Don't be afraid to ask or talk to the professors. They are great and want to help you out the best they can!
I wish I had known the atmospheres of the respective dorms better (Olson for pretty, often vain women; English for down-to-earth, motherly types; Gerig for the more nerdy variety, etc); I ended up in Olson and had a basically miserable dorm experience for the one year I lived there.
I wish I had known the opportunities available to me with respect to non-classroom activities better. I wish I had known that it was so easy to participate with other music students in ad-hoc activities.
I wish I would have realized that not everyone comes from the same background as me, and therefor not everyone will always be ok with my point of view.
It gets cold during the winter, so pack on the layers. The cold leads to some of the best and most fondest memories though too.
I wish I had known that I don't have to buy textbooks from the bookstore and that students can sometimes share textbooks.
I feel like I came in with the right amount of knowledge that I didn't have stereotypes already formed.
I really can't think of anything I would have liked to have known. My experience has been pretty good.
I wish I could know which classes to take and which not to take and with which profs.