Airlines have special ticketing codes that help identify what type of passenger you are to them. Each airline has different codes but some are universal. The Y code, for example means that you paid full fare for your ticket and it is fully refundable. Y class ticket holders will be the first passengers considered for an upgrade.
Pay for a full-priced coach ticket when you book and make sure that your ticket has the Y code on it. Airline code information is available online.
If you are traveling for your honeymoon or anniversary, let the agent at the gate know. Be conversational when bringing it up and let her know that you would truly appreciate anything she could do to help make the start of your trip memorable.
If you want an upgrade to business or first class, look important. Airlines are constantly seeking out corporate business accounts and want to attract business people who will pay a premium for their seat and services.
Looking like a busy executive when you ask about an upgrade will likely increase your chances. Men should go to the airport wearing a business suit, while women should also wear appropriate business attire.
While flight attendants are trying to stuff the last bags into the overhead compartments, find a way to get to the front of the plane to see if there are any empty seats in business or first class. If you find empty seats, go back to your seat and call the flight attendant.
Politely ask if you can move to the available seat. Unless your seat is broken, you must have a good reason to want to move, such as body odor from the person sitting next to you, or an over-sized person taking up some of your seat space.
If a family member or friend works for the airline you are flying, especially at the home airport you are flying out of, ask them to hook you up with a free upgrade. If you can't get this worked out before you get to the airport, let the gate agent know about your connection.