~Organize your locker, backpack, notebooks, etc. The neater you are, the easier it will be to get between classes.
~Organize your studying. There is much more work in middle school than there is in elementary, so be sure to keep on top of it all.
~Be positive. When you enter a new school, you get a chance to re-identify yourself. You can do anything you want to do, so go ahead and try out for theatre, drill team, or chorus- whatever floats your boat. But keep in mind that you are defining your social like for the next three years when you act. Don't be mean to people or afraid to talk to them.
~Retain a sense of self. What are your moral values? Do you want to be religious, stand up for gay rights, start kissing boys? Middle school is usually when you make all these decisions, but you have to go in with a few basic rules. For me, I decided I believed in my G-d and I wouldn't get involved with guys until after middle school. If you doubt something, don't do it, because you might regret it later. For example, my friend first had sex in 7th grade, and she's been regretting it since 8th grade- we're in 11th grade now. If "all your friends" are doing it... well, there's plenty of time for stupid mistakes in high school.
~Go ahead and be in those cliques if you want to. Everyone says stay away from the mean, materialistic girls, but hey- they can be fun some times, as long as you don't get carried away, and remember that it's not the end of the world to stop being friends with them.
Good luck! Email me if you ever want more advice: firstname.lastname@example.org
ETA: It's very useful in the future if you start activities you enjoy like youth groups, sports, and other extracurriculurs now. It'll help you get into college. Oh, and practice essay writing. You'll DEFINITELY need that one alot later on.
11th grader, 8th grade bro, 6th grade sis