How to Buy Plane Tickets on the Same Day of Departure

On a recent excursion I found myself in Washington, DC, and I was ready to leave. There were just two problems: I didn’t know where I wanted to go next, and—perhaps related—I didn’t have an onward plane ticket.

This situation wasn’t that unusual, though. I’ve frequently found myself buying plane tickets very close to departure, and usually getting a good value despite the common perception that they are outrageously expensive.

The secret? Use your miles instead of cash, because last-minute a...