From Wisebrother.com:
Are you flying somewhere for the holidays with your significant other? Before you buy tickets, here's a trick that might make your trip a little more comfortable.
Someone came up with a hack that lets two people flying together get a whole row of seats to themselves. It's not foolproof, but it's worth a shot. Here's what to do . . .
First, look for rows with all three seats open. If there aren't any, or the flight gets fully booked, you're out of luck. But that's step one. And book two of them . . . the window seat, and the aisle seat.
Maybe you've tried that part before, because obviously the middle seat is the worst. Those are the last seats people pick. And even if someone does end up between you, they'll probably switch seats, so you can sit together.
But this second part is where the magic happens: If possible, choose seats in row THIRTEEN.
A lot of people get superstitious, especially when they know they'll be 30,000 feet up with no control. So if you book a window and aisle seat in row 13, there's even less of a chance someone will book the seat between you.