Pay for part
If you're willing to cough up a bit of cash or use your frequent-flier miles, you also may be able to get upgrades at cut-rate prices.
If seats in business or first class are vacant, they may be offered to economy-class travelers at check-in, according to Frey.
"You have to be on the lookout when you check in," he said.
Another option is using frequent-flier miles. While you may encounter turbulence booking a flight using miles, it's often easier to book an upgrade, Frey said. The miles are "almost like currency."
Whatever your strategy for trying to earn an upgrade, Frey said the most important thing to keep in mind is, "There's just not as many seats to go around these days."