Be an awards member: This is always going to come in handy, especially if you’ve racked up miles and a high status. Award members often get special treatment that others don’t. Sometimes you may have to use those miles to get a seat upgrade, but other times you may just get it by telling the agent you’re an awards member.
Ask nicely: Just ask, but do so nicely. If they deny you, sit somewhere that the agent can see. This will especially be to your advantage if you’re traveling alone. People who travel solo are much more likely to get a better seat then someone traveling with a spouse or family. Ask nicely if you can get a better seat. If it doesn’t work at the ticket counter, then wait until everyone has boarded and you can see what seats may be available on the plane.
Dress up: As in, don’t wear a tank top, ball cap, and gym shorts. This is especially the case if you want the chance at getting a first class seat. The better you’re dressed, the more likely you’ll be picked over someone else for first class. You don’t have to have a complete suit on, but dress professionally.
Be early: Be early, but you won’t be saving time at the airport if you’re too early. Sometimes you can snag a better seat if you get there last-minute and some seats are still being assigned, but usually your best chance will be to arrive early. Since you’re early, you’ll probably be much more composed anyways, allowing you to look the part, and ask nicely for a better seat.
Consider your flight wisely: This is something you can do before you get to the airport. Unless you’re looking for cheap flights months in advance, picking a seat can be slim pickings. However, choosing a flight time that is during off-peak travel times can improve your chances. Consider red-eye, Saturday, or mid-week flights when planes aren’t as crowded.