I have a (good) friend who improves his chances to sit beside a free seat (on not full airplanes) by:

Having a second seat reserved (at his side) on his wife's name (with a full-fare-unrestricted-one-way-ticket), cancelling this seat just at the very last moment (later on cashing in the full-fare-unrestricted-ticket).

He says, that sometimes, when the flight is really full, and the free seat will "disappear" anyway - he/she might even volunteer for giving up the seat (and cashing in what ever they get for doing this favour to the airline ...).

As I said, I would never do this, but I can't controle my friends ...

(from the guerilla-handbook)