If you’re into the numbers (which I assume you are if you’re day trading) then check out this book – The Mathematics of Money Management: Risk Analysis Techniques for Traders.
It’s actually quite expensive now ($72 on Amazon) – I don’t think it was that much at the time, but it’s far and away the best book I’ve read on this topic.
It will help you make objective, logic-based decisions about how much to trade and re-invest based on your risk appetite and overall portfolio.
Trade2Day