We can all thank Spirit Airlines for the innovation in travel that is seat-based pricing. So now, many airlines have a system where bulkheads, exit rows, seats near the front, and aisle seats cost extra to book. Meaning if you’re over 6ft tall and don’t want to feel like Buddy the Elf every time you step on a plane, it can cost you up to $80 a flight. And that doesn’t even come with open bar.
HOWEVER, since we’re all not suckers and don’t opt to pay a premium for a seat that could be comfortable, those “premium” seats don’t always sell out. So arrive at the airport early, and wait in the line to talk to an actual agent instead of checking in at the self-help kiosk. If you’re friendly, smile, and talk to the agent like a human being, asking politely for one of the better seats is a pretty high-percentage shot. It’ll involve a little bit more of your time, but it's absolutely worth it if you need extra leg room.
The other (admittedly risky) way to get a premium seat without paying for it is to check in on the late side. Since most people will decline to pay for seat upgrades, exit rows and other premium seat locations are often the only ones left for the last few passengers to check in, leaving the airline no choice but to place you in those seats for free. But you should only use this strategy if you're feeling lucky, otherwise you'll probably get stuck with a middle seat in the back of the cabin, next to the bathroom.