It's the morning of your very first award flight. You paid $0 in airfare plus $5.60 in TSA fees to cross the country, and you're feeling great. That severe thunderstorm approaching quickly from the south won't be a factor -- your plane will leave on time, 15 minutes early, even, and you'll be comfortably seated in an extra-large recliner as you sip only the finest California sparkling wine. Nothing will get in the way of you and a great flight. You didn't buy a First Class ticket, but no matter -- you're riding up front today. Because there's three empty seats and you showed up at the gate with a gigantic smile and a three-piece suit.
Yeah, fat chance. While that strategy may have been successful a decade or two ago, technology is working against you. Sophisticated airline IT systems track empty seats and work to fill them with the most appropriate passengers. "The computer" pays no attention to your attitude or appearance, making decisions regarding who to upgrade based only on what a customer paid for this particular flight, whether or not they applied a cash co-payment, miles or an upgrade certificate to the reservation, and how much he or she spends with the airline each year.
Of course, free (and many paid) upgrades apply only to domestic or regional flights on US airlines -- if you're flying overseas or with a foreign carrier, you may not be able to upgrade at all, unless you're willing to pay the full fare difference between the current price for a First Class seat and what you previously paid to book. Many airlines in Asia or Europe don't upgrade any of their passengers for free on any flight, even a short hour-long hop. With other airlines or international itineraries, an elite customer who flies hundreds of thousands of miles each year may end up traveling in Coach with a dozen or so seats still open in First.
Like carriers abroad, a US airline's primary objective is to sell every seat on the plane at the highest possible price. They're just a bit more generous with filling empty premium seats just before the plane goes out. When it comes to free upgrades, tickets for non-revenue passengers, such as friends and family of an employee, come in at the bottom of the list of priorities, but an award ticket in which the airline (or one of its partners) collects next to nothing is a not-so-distant second. On flights with tremendous demand, such as routes with limited competition, an airline will sell out almost every flight while maintaining a very high ticket price for both First Class and Coach.
But on routes where an airline competes with many others, and during periods when the load of passengers traveling for business is relatively light, such as during a holiday or over the weekend, there may be a handful of open seats in First Class even on the day of departure. The airline will do its best to sell paid upgrades to Coach passengers during the days leading up to the flight, but if empty seats remain, elite passengers are first in line to receive them for free, at various levels of priority based on how much they flew in the previous year.
If all eligible elite passengers have been upgraded or only some lower-level elites remain, the airline may often offer any Coach passenger an upgrade at check-in online or once they arrive at the airport. Some airlines, such as Delta, have even experimented with allowing passengers to bid for an upgrade, letting you type in the amount they're willing to pay to fly up front.
Assuming you didn't pay for that upgrade at check-in or you weren't eligible (which you may not be when flying on an award ticket), and there are seats remaining, the gate agent may be able to assign them to remaining passengers, including yourself. Typically, this only happens when Coach is overbooked, in what's called an op-up, or an operational upgrade. In this case, particularly if the flight is running behind, a gate agent may choose passengers at random, Their objective at that point is to get everyone a seat so the flight can leave as quickly as possible.
If the stars align for your flight, you could, in this very rare situation, get that upgrade for free. Being dressed nicely certainly wouldn't hurt, but your attitude with the gate agent will have a much greater bearing. Be patient and polite, and consider handing over your boarding pass for the gate agent to hold on to and sitting within their line of sight. If there's a seat at the last minute and they need to move someone into it quickly, that someone could be you!