I don't know too much about the kelly criterion, but going by the other answers it sounds like it could be quite risky depending how you use it.
I have been taught the first thing you do in trading is protect your existing capital and any profits you have made, and for this reason I prefer and use Position Sizing (PS).
The concept with PS is that you only risk a small % of your capital on every trade, usually not more than 1%, however if you want to be very aggressive then not more than 2%. I use 1% of my capital for every trade.
So if you are trading with an account of $40,000 and your risk R on every trade is 1%, then R = $400.
As an example, say you decide to buy a stock at $10 and you work out your initial stop to be at $9.50, then our maximum risk R of $400 is divided by the stop distance of $0.50 to get your PS = $400/$0.50 = 800 shares. If the price then drops after your purchase, your maximum loss (subject to no slippage) would be $400.
If the price moves up you would raise your stop until your potential loss becomes smaller and smaller and then becomes a gain once your stop moves above your initial purchase price.
The aim is to make your gains be larger than your losses. So if your average loss is kept to 1R or less then you should aim to get your average gains to 2R, 3R or more. This would be considered a good trading system where you will make regular profits even with a win ratio of 50%.