Continuation bet size and optimal frequency?

I generally make continuation bets of half the pot. Is it wise to vary the amount some times?
Also, I go on judgement when to make the bet but is there a mathematical optimium frequency such as the game theory approach used in Theory of poker?
I tend to make the bets when i'm out of position rather than in position, where I sometimes take the free card and if it's no help I might make a continuation bet on the turn.
Would like to hear your thoughts.