There's no one-size-fits-all trick for landing an upgraded airline seat these days—and that can mean more opportunities for travelers willing to crunch the numbers.
Tallying enough miles to crack an airline's elite status tier used to be the quick way to free upgrades. But smaller first-class cabins, fuller flights and increased airline efforts to sell those prime seats has eaten away at that advantage, said George Hobica, publisher of deal site AirfareWatchdog.com. "I've seen flights where there are 20 elites on the eligible [wait]list and not one person gets upgraded," he said.
(Looking for hotel, cruise or car rental upgrades? Check out the video above for tips.)
But the elites' loss may be a gain for other travelers willing to put a little effort and cash into a better seat. Some options to consider:
Buy economy, redeem miles to upgrade. It takes fewer miles to upgrade than redeem outright for a seat, but that doesn't mean it's always a bargain. Airlines often charge a fee in addition to miles required, and that fee varies by what fare class you originally bought, said Brian Kelly, founder of ThePointsGuy.com. (The cheaper the ticket, the more you'll pay. A flight to Europe on United, for example, might have a fee of $300 to $550. Only two fare classes have no upgrade fee.) Availability can also be uncertain, with some airlines terming the upgrade a "request"—so check for that confirmation email.
Pay cash to upgrade. "I always tell people, before you assume that the mileage upgrade makes sense, log into your account and check," said Kelly. Airlines are offering more opportunities for fliers to pay cash upgrade fees, not unlike how you might add on an extra legroom seat. The rate might be a better deal compared to a cash-and-miles combo, he said.
Read MorePlan ahead to cut vacation fun costs
Redeem miles. No upgrade finagling necessary: Just trade miles outright for that business- or first-class seat, said Stephanie Serino, a luxury travel consultant for Tzell Travel Group. "Using your miles to get international business-class tickets is by far the best value," she said. "I don't understand why anyone uses miles to book coach, domestic tickets." It's not always a splurge, either: Look for "saver" options on less-crowded flights, which can require half as many miles. Partners in an airline's alliance may also have cheaper redemption options available.
Bid for upgrades. A number of airlines—including American, Etihad, Lufthansa and Air New Zealand—auction available seat upgrades. "The highest bidder gets the seat," said Kelly. But bid strategically to avoid wasting miles, he said. Forums at FlyerTalk and other sites have plenty of posts from travelers dishing on how many miles it took to successfully land a seat on a given route.
Read MoreReward strategies to stack up deals
Just buy the better seat. Sometimes the price gap between economy and the next-best class isn't that substantial—especially for travelers who routinely pay for extra-legroom seats and other add-ons like checked bags and in-flight meals, said Serino. Instead of paying for a few extra inches of space, for a bit more money, you might be able to snag a comfier seat (with extra leg room), better (and free) food and more entertainment options, she said. There may even be extra value like multiple free checked bags or priority security lines.
Procrastinate. If you're not sold on the value of an upgrade or there aren't any currently available, pay attention as the flight approaches. Upgradable seats often "magically appear" three to five days out for travelers checking online seating charts, Hobica said. Airlines may also email offers to upgrade or make a note of availability at check-in—occasionally at lower prices than previously offered.