For systems that work very consistently a simple system where you trade a fixed fraction of your equity may be used.

The Fixed Fractional strategy starts out with a fraction to use, say 10%, and always trades this same proportion of your equity. If you win on a trade, your next trade will be slightly larger in dollar (or contract) terms than the last, as you will trade 10% of whatever your new account balance is.

An aggressive trader will trade a higher proportion of his account per trade than a less aggressive one, starting with a fraction like 30%, as opposed to 5%. Naturally risk of ruin is substantially increased by larger trade sizes.

If you buy contracts rather than a dollar amount, you buy the number of contracts that most closely comes to the fixed fraction of capital, without exceeding that limit.

This system is a good one because it allows your success to determine how much you bet. It is unlikely that most people would want to keep trading the same small amount as their equity grows, so this system gives a chance to increase your trade size if you are trading well, or progressively reduce it when you are not. It requires a very good trading system to work though, one that makes money on a regular basis, without too long a losing streak.

The downside of a Fixed Fractional strategy is that risk increases rapidly as the account grows.  When the account doubles the drawdown doubles as well, when it doubles again drawdown increases by 400% of what it was at the start.  Many traders prefer a more constant, or even decreasing drawdown, which means a decreasing fraction of the account is risked in each trade.