I have been doing a little bit of Microstock here and there for a few years but never really applied any analytics to it. Four months ago, I set out with the goal of tracking my progress and quadrupling my Microstock income purely by spending four hours a week in the first month, two hours a week in the second month, and one hour per week in the third and fourth month on it. My first four hours were spent on researching various Microstock sites, creating new accounts and developing a spreadsheet to track my progress. The second four hours were spent tagging all the images with Metadata and uploading them. There is a nifty tool called ProStockMaster that allows you to submit to all the major Microstock sites at once. During the four months, I almost doubled my portfolio purely out of pictures I already had but never used ranging from objects to lifestyle (a rough average of what's out there). I did not reach my goal of quadrupling my income, but I increased it by a factor of 3.5, which is pretty good.
On the way there, I found out which sites actually make you good money and which ones don't. I also made several accounts to track different factors that aren't part of this write-up, so here goes the summary ranked by awesomeness (which is how much money they made me). Just so you know, I have a degree in economics and finance, so I actually know about this stuff.
These are the six Microstock sites I would go with. The other ones had no or only very minimal payouts below 1% of my monthly income. They were: CanStockPhoto, BigStockPhoto, PantherMedia, YayMicro, CutCaster, and StockFresh.