Many of you would have heard of the Martingale system of betting (trading). If you are winning, keep betting the same amount (position size). At your first loss, you double up, and continue to do so til you win back everything you've lost, and start over. It's not actually doubling, (there's a simple formula), but you get what I'm saying. You might get lucky if you don't trade for too long, but the likelihood of not having the required capital to cover your nth losing bet is high. Not recommended.
Reverse Martingale can be used successfully with share trading, I believe. If your first bet wins you increase your bet size. The more you win the more you increase the increment (with some limits obviously). The idea behind this system is that it makes use of the 'winning streak' phenomenon. First loss will cause you to reduce your bet size. Lose again, reduce it again... and so on.
Probably best for high frequency trading systems.