3.5yo is a good age to start understanding about rules and acceptable behaviours. Start early brainwashing, er, teaching about why she needs to keep the seatbelt on ("It's like in a car. So that you don't bump or get hurt), why she shouldn't bump the seat ahead ("It's very very uncomfortable, and the lady ahead of you may hurt her back"--a bit of exaggeration won't hurt. Show her how it feels if someone behind HER kicks HER seat.)

We aim for the "lose a battle and win the war" strategy. I don't really consider this as bribing, but let my kids have a small say (choice of snacks, toys, lights on/off, shoes on/off) but remain firm on the big issue.

In case of our kids, boredom, tiredness, hunger tend to trigger hard-to-control situation, so we try to pre-empt them. It definitely helps to have an individual screen (though my kids find it frustrating to keep the headphone on sometimes), give her an idea how long the flight is, take her around for "an adventure walk" to the back of the plane (another reason to request an aisle seat for the accompanying adult, in addition to the loo visits).

Try to keep YOUR travel-related stress low. Kids have amazing way of picking up on parents tension and magnifying it. Also, just as a social grace, we offer apologies (sometimes in advance.... or in anticipation?) to other passengers around, but 80% of fellow passengers tend to be quite understanding/amused by the kids.