As your account grows you want to make bigger trades and vice versa as your account shrinks. Fixed fractional is a little too aggressive, though. While you can stomach a 50% drawdown with a $10k account, you would really hate to see that with a $1m account.
But Ryan Jones' fixed ratio is a little too conservative, though. While it makes sense to double your account to trade another contract at $5k, it doesn't make sense to have to double your account to trade another contract at $1m.
Somewhere inbetween these extremes is good. Here's a suggestion...Bank 1/2 of all new profits when you hit a new peak in equity and trade fixed fractional on the remainder.
Or even more sophisticated... Bank all new profits once your account exceeds half (or a third or whatever) of your net worth and trade fixed fractional on the remainder. With this method you can let your account grow larger and take on more risk if your other investments are doing well.