Not security related, but don't plan on having any open space on the plane. I think every time I've flown in the past 3 years they flights are either completely full or have like one open seat. Doesn't matter when or where I was flying it is packed.
If you are taking a roller suitcase as a carry-on follow the damn directions from the flight attendants. It's amazing how much better the bags fit when everyone puts them in correctly.
This is a very good point. Planes are so full. I always pick my own seat to make sure that I get what I want. Leave it to the system and you may well get a middle seat.
I always use the kiosk when I arrive, even if I have checked in online, because sometimes seats have opened up. For instance, they often don't assign seats in the emergency exit rows until the day of the flight.