you're fine to fly up until 36 weeks-barring complications-I flew at 30 weeks to Alaska-8 hour flight-and that was fine. Bring a water bottle that is refillable as you can not bring fluids through security anymore-and walk during the flight as much as possible-just pace the aisles! Flight attendants will understand when you tell them you are prego. Definitely bring snacks as well because flights rarely serve food anymore and you want to stay full with healthy snacks.
If you are obviously prego, you may want to bring paperwork showing how many weeks you are and your expected due date since airlines won't let you fly past 36 weeks and they may want proof.