Considered in a vacuum of perfect randomness, rock, paper and scissors are all equally viable and equally strong options when playing a good old fashioned game of Rock-Paper-Scissors. Therefore, if you were playing against a computer that approached each game with a fresh batch of absolutely no human emotions or expectations, your best bet would be to play each option exactly one-third of the time. But since you're probably not playing against a computer (right?) you have to take into account your opponent's conditional response—or, how previous events will shape his or her behavior.
To examine this, researchers at Zhejiang University in China had 360 students, broken into 60 groups of six, play 300 rounds of Rock-Paper-Scissors (over the course of a presumably very monotonous couple of hours). Their findings are represented by a series of increasingly complex graphs and equations here, but if you're concerned about the practical application of winning trivial hand games, check out this video produced by the nonprofit Mathematical Sciences Research Institute.
The video details two key findings from the study that help you decide what to play next after an initial game of Rock-Paper-Scissors. In each game, there are two possible outcomes: you win, or you lose. Since we are trying to predict what the other person will do, it helps to consider this from their perspective: they win, or they lose. If they win, they are statistically more like to re-play whatever they just won with than the other two options, and you should react accordingly. So, if you play paper and they beat you with scissors, chances are, they will play scissors again—so in order to win you should play rock.
Let's consider the other option—that they lose. In this case, they are more likely to behave in the next game as you did in the previous scenario, which is to say, play the thing that beats the thing they lost to. Knowing this, you should play the thing that beats the thing that beats the thing they lost to (you follow?). Or, if you play rock and beat their scissors, they will most likely play paper, assuming you will repeat rock, so in turn, you should play scissors. Since there are only three options, you can simplify this by just playing what they played the previous round.
Just hope your opponent didn't also get a look at this study.