I think one of the major ones that I would have told myself is to make sure you have fun, but not too much fun. If you know you're going into town with some friends for some coffee and to pick your lunch (at 2nd period instead of at lunch time because you think it's going to rain?) but you do know you've got an essay you haven't done in for 5th period... Here is what you do: you give your money to your friend(s) that are going, you stay behind, look after their bags and get writing on that essay and tell them to meet you in the library!
You do need to know the work/life balance at sixth form which is hard at first, because you've been told you must keep up the work at all times and it will get overbearing and etc (which is true) but don't feel guilty if you are planning to go to the cinema on Saturday. If you know this is happening, then make sure you do most/all of your homework during the week prior to that Saturday. (Because let us all be honest: no one likes or does any school work on Sunday and if it is, it's just copying h/w you asked your friend to send a photo of). Doing your own homework helps you understand what you are learning and it also helps you retain the information if you look into it deeper. Trust me when you're given the homework and you're like "omg, this is so pointless such a waste of my life"... 8 months later you'll be wishing you had done it when it's crucial to revision!
So basically, have fun, but not too much fun!