Learn the airline's ticketing codes before you go to the airport. Codes determine what type of passenger you are to the airline. There are different codes for each airline but a few codes are universal. For example the Y code means that you have a fully refundable ticket that you paid full fare for. Y class passengers will be the first to be considered for an upgrade. Make sure that your ticket has that code if you paid that fare, or kindly ask the ticket agent at the airport if they could add the code to your ticket to increase your chances of being upgraded. Airline code information is widely available online.
Inform the gate agent if you are traveling for a special occasion. If you are flying for a honeymoon or wedding, be sure that the gate agent knows that and that you would appreciate anything they could do to make it more memorable.
Dress like a VIP. Airlines want corporate business accounts and the accounts of busy executives, so if you look like you are an important person when you ask for your upgrade, you may increase your chances. Men, wear a business suit, and ladies wear a smart dress or pantsuit.
Use your family connections. Ask a family member or friend that works for the airline to hook you up. If you are not able to do that before you get to the airport, inform the agent of your connection.
Check the first class cabin just before take-off. Once everyone is on the plane, but it has not left the gate, casually walk to the front of the plane to see if there are any seats available in business or first class. If there are, go back to your seat, call the flight attendant, and kindly ask if you can move to the available seat. You will probably have to have a good reason to do so. Legitimate reasons include a broken seat, body odor from your neighbor or sitting next to an over-sized person who may be taking up some of your seat space.
