You know those barf bags in the seat pocket on the aeroplane? They're good for more than a little vomit. As Instructables user wilgubeast points out, they make for a great way to keep the seat next to you empty.
Impossibly relevant photo by Mike Hartnett
Most airlines assign your seats ahead of time, so placing a barf bag on the seat beside you isn't going to do much unless you're flying on an airline that doesn't. But it's not the aeroplane that makes the barf bag unappealing. It does that all by itself. Take it on the bus, to a movie theatre, or anywhere you might want to keep a seat empty. Maybe it's a little cruel to make someone else stand when there's technically a seat empty, but there are those times when you just really need your space or you're just saving a seat for a friend. With aeroplane barf bags comes great responsibility, so use it sparingly and be conscientious.
Hold a seat [Instructables]