Guide to Paper, Scissors Stone
1 - Players usually lead off with either Scissors or Stone, making stone the best choice to go with first (as you'll blunt scissors or tie with stone).
2 - Sub-consciously, players tend to copy your last move, so stay one step ahead by choosing scissors if you chose paper last, stone if you chose scissors and scissors if you chose paper.
3 - Note taking is crucial - do they tend to mix it up or stick to their favourites? Do they always change choice from last go, or often choose the same 2-3 times in a row?
4 - Mix up your own play - randomise the speed of your response etc.
5 - Yes, i'm taking the piss.