The Best rock, paper, scissors strategy is this:
First off, make sure you play 2 out of 3. Any less and you might lose on dumb luck, any more and the poor fool whom you're playing against might catch on.
So what's the strategy?
Throw rock every time!
The first time, you might lose. Don't be discourged though, because your opponent is now quite sure that you're not going to throw rock again. In fact, he's pretty sure you'll throw scissors, since they beat paper, which he just threw to beat you. In any event, he'll definitely not throw paper again assuming he did beat you the first time.
Basically, it's safe to assume that the opponent highly doubts you wouldn't adjust your throw based on how you did in the first few throws. So chances are he'll mix it up. The important is not to wuss out. If you just throw rock every time, you'll win 95% of the time.
However, don't play just single round (since you could just lose that one) or 3 out of 5 (sice they'll probably catch on). Also, don't play against someone who knows you're going to use the strategy.
Good luck!