I can't believe it hasn't been mentioned already but join a frequent flyer program.
Which one? Well, it depends if you know if you'll be doing much flying in the future. If you're not sure, I would think that United's mileage plus (www.mileageplus.com) would be a good idea. It's free to join and you'll get your own frequent flyer number. Make sure you quote the number when you check in for your flight.
The other tips I would give are the usual ones. At checkin, ask if the flight is busy and, very politely, ask if they're needing to upgrade anyone. If they're not then ask if any of the bulkhead or emergency exit seats are available. This way, you'll get more leg roon.

Enjoy your flight!