Advice from a 4 *:
As far as mental preparation goes, It's almost impossible to prepare for what you get thrown in to. The transition, the non-stop yelling, the quick thinking and the belittling all seems to come at you at once, especially since the cadre can modify their training to how they see fit for your group. Sometimes that's standing at attention for long periods of time while "studying" knowledge. Nothing can prepare you for that, it just sucks. As long as you come in with an open mind and are prepared do anything with a good attitude you should do well.
Physically, in my squad at least, they emphasized the team aspect pretty heavily; the whole "you're only as strong as your weakest link" type of thing. It really helps if you're in decent physical shape so you can help out your squad mates when they're "breaking" during push-ups, runs, flutter kicks, etc. One huge misconception that I had going into basic is that my performance from basic wouldn't affect me during the academic year. Well so far I've seen my basic cadre every day, since I live with them, and their first impression of you is obviously going to be from BCT. They may not be your cadre during the year, but they play pretty big roles in determining your stratification in your squad which weighs heavily on your MPA. Just stuff to start thinking about before you come, but enjoy the rest of the time you have at home with your family and friends while you can.