Tactics for getting the elusive "free" upgrade
Dressing well is not the ticket to ride some hope. It is, but even so, you are not getting an escort to the front of a plane if you are wearing cargo shorts, a tank top and flip-flops. Most people don't even want to sit next to you in this case, let alone upgrade you.
Say something as simple as "if you are upgrading passengers on this flight, I would like to be considered". Inserting the word "please" won't hurt you either.
BE ON TIME AND HAVE GOOD TIMING
Showing up late to request an upgrade when an agent is just trying to get everyone checked in and in the air ins't going to work. Do agents the courtesy of making your request with plenty of time to spare before the flight, and when no one else is competing for their attention.
Being overly demandingor demeaning just inspires agents to pick someone elso to upgrade if the opportunity arises. And don't waste everyone's good time and good will if you know that you are a poor candidate. If you are travelling with your whole family, have a pet lobster in a cage as your carry-on or purchased a ticket for an extremely low fare, you probably don't want to spend your energy demanding upgrades.
Next, making it happen...
Even though seats in business class may also be empty, the airlines don't usually upgrade people for no reason. If the flight is full, your chances are better. Air-lines carefully plan how much they oversell flights and their inventory departments are not upset if people need to be upgraded to accomodate everybody on the flight. Therefore, on a full flight the airlines sometimes are forced to upgrade people. In this scenario, if you have a good story, you may be lucky. Remember, of course, that business or first class may already be full from prebooked elite-level upgrades.
Tell the agent that if they don't need your seat but they do need somebody to upgrade, you'll be happy to volunteer for that. Small chance, but worth a try. If they end up needing your seat for someone else, ask whether you can be upgraded on the next flight.
Again, airlines don't generally upgrade people for no reason, but if they have caused you a problem, that may be reason enough. Also, ask about availability at check-in, particularly on international flights, where the check-in agents sometimes have more control over the seating chart. Then, if seats appear to be available, check-in again at the gate. The final, "miracle" upgrades always happen at the last minute, when all passengers are checked in and any remaining availability becomes clear. Make sure you are within earshot of the gate desk, although hovering over agents is not recommended.