Originally Posted by okok

It's like poker man or horse racing, figure out what you can afford to lose, and put your bankroll down in small percentages. ROI positive on this bet? Put more into it... No return there? Stop making that bet. It's just making lots of small investments and sticking with what's hot.

You also need to consider whether what the traffic is hitting is best suited to them... maybe one source would do better with a different landing page... so many little factors you can play with.