Acquiesce to the passenger in the middle seat. Why not throw the middle man a bone? It's the least desirable seat, after all...
Stake your claim. It's not your fault he waited to buy his ticket/ check-in so late he's stuck there. Why should he be rewarded for being lazy?
Wear beach gear
Air Marshals aren't fashion police. Who are you to tell me how to dress? Comfort should be king, and this is my best 'going out' tank, anyway.
Follow the '50 feet from the water' rule. If you can't see the rippling waves or smell the chlorine, tank tops aren't appropriate, especially considering the close quarters of coach.