Getting a good seat on a flight is awesome, but getting one with no one sitting next to you is even better. If you're lucky enough to be on a flight that isn't filled to the brim, here's a trick that'll increase the possibility of having an empty seat next to you.
If you're booking tickets with a friend or family member, you probably look for two seats together, but a better strategy is to book an aisle and a window seat in the same row. People don't like to sit in the middle, so that will probably be one of the last seats booked -- meaning if the flight isn't full, you have a chance of having an empty seat next to you, with room to stretch out a bit. And, if someone does book that middle seat, they'd be more than happy to trade with you so you can sit next to your travel buddy (since, again, no one wants to sit in the middle).
If you're travelling by yourself, you can still use this strategy by booking a seat in a row where either the aisle or window seat is already taken, but the middle is not. Don't choose one in an empty row, or the other two seats might be booked by a couple travelling together. Again, it doesn't work every time, but on the rare occasion that flight doesn't fill up, you can relax in a bit of extra space.