Tell the truth.
Lying is a big problem for many fictional middle schoolers. Often kids lie to impress a potential boyfriend/girlfriend, like Kevin Spencer who lies to get out of class in order to get closer to his crush, Tina, or Cici Reno, who pretends to be someone else on Twitter in order to get the attention of the boy she likes. Other characters lie to manipulate situations, like Avery from Fake Me a Match by Lauren Barnholdt, who tries to win over her stepsister by rigging a matchmaking service to pair her with her crush, or Sadie from Peanut, who invents a peanut allergy in order to make herself seem more interesting. There are even characters who lie to cover up what they've done, such as Thad in How to Break a Heart. In all of these situations, though, the lies snowball to the point where they take over the characters' lives and eventually, the awful truth comes out in an awkward and difficult confrontation.
Make friends who share your interests.
I have to admit that the ease with which girls find like-minded friends in fictional middle schools is not always believable. I really didn't have friends during middle school at all, so this idea that you could walk into the cafeteria (or detention, or an after school club meeting) and immediately find your tribe sometimes makes me roll my eyes. But it's definitely worked in series like How to Survive Middle School, Annabelle Unleashed, The Odd Squad, The Snob Squad, and Nerd Girls. Personally, though, I think Jamie's friendship with Isabella in the Dear Dumb Diary books is much closer to the truth for most middle schoolers.
Don't bring your diary to school.
Harriet from Harriet the Spy learned the hard way what happens when you write down blunt observations of your classmates and those words fall into the wrong hands. Still, that hasn't stopped other fictional characters from keeping diaries and bringing them to school. In both Mackenzie Blue and This is All Your Fault, Cassie Parker, diaries are stolen and shared with the very people from whom they should have been kept most secret. If a middle schooler is going to share her innermost thoughts, it is best to do so at home, and then hide the evidence!
Don't over-emphasize popularity.
Many, many protagonists in novels about middle school worry about their popularity, but whether they lose it, experience it temporarily, or can't even achieve it in the first place, they eventually realize that being well-liked by many people is not as satisfying as have one or two close friends. This common theme runs through many popular series: Dork Diaries, Popularity Papers, The Classroom and The Winnie Years, as well as in titles like Always, Abigail, Mission (Un)popular, and Pack of Dorks.