Some thoughts on varying your position size

In my own trading, I use fixed fractional position sizing - that is, while the monetary value of risk per trade varies as my equity goes up or down, I risk the same amount in percentage terms.

A while back, a good trader friend of mine experimented with varying the percentage risk per trade based on a look back period of performance. 

Both methods have been used by successful traders and Market Wizards. Both have strengths and weaknesses.