Just Say "Ouch"

Few things are more irritating than when someone sits in front of you on an airplane and immediately reclines right into your lap. When you first sit down in your set, gently press your knees into the seat back in front of you, giving the illusion (or reality for tall people) that your legs are occupying all of the available space. If the person reclines, they should immediately feel the resistance from your knees and stop reclining. If they keep reclining, say "ouch" loudly enough for them to hear you. Usually they will turn around and give you a dirty look before un-reclining their seat. Dirty looks aside, you win because you have the room your ticket paid for.