I always keep my knees wedged right against he seat in front of me for the first 20-30 minutes of the flight. That way, when the person in front of me tries to recline, their seat won't move. They usually fiddle with their button for a few seconds and then give up, thinking that their seat is broken or doesn't recline. They rarely try again throughout the flight.

Sneaky maybe, but it seems to work like a champ!