MacWorld has seven tips for would-be microstockers (see below). It's always helpful to read other people's summaries of the things that they find important. This is a decent list of basics.

1. Understand how it works: "...Selling stock photos is not easy money. It will take time to build up a decent sized portfolio (at least 100 pictures) before you see any sizable return..."
2. Think like a designer: "...Designers want photographs with ample amounts of uncluttered negative space so make sure you leave room for a headline or other text..." [Click the title above for more]

Shutterstock and iStockphoto are now allowing video submissions to sell short video clips to local news stations, documentary producers, and the like. If you can produce video footage that would be useful to these types of projects, it could be well worth your while to upload them to Shutterstock or iStockphoto.

This is a short sample clip I took to submit to Shutterstock. I can imagine local newscasts  using this clip in a story about recent Supreme Court proceedings, or documentary filmmakers using it in a commentary about a particular case, and so on.

Dreamstime offers four extended licenses which could increase your earnings significantly: Increase Max Copies, Web Usage, Print Usage, and Sell the Rights. Each of these extends the ways in which the image can be used and pays a much higher premium. If you sell the rights to the image, you no longer control it and cannot sell it elsewhere, so use it with caution--noting that you can make a large chunk of change using this option (you can set your own price). Further details are below.

Shutterstock, Dreamstime, and iStock allow you to sell your vector illustrations on their site in the same manner as the digital photography. This means you can create imagery in Adobe Illustrator, Freehand, Xara Xtreme, or Inkscape (free download). You earn money for every download from their subscriber base, with additional incentives for referring buyers or sellers to the site.

This sample image is one of my most popular vector images on Shutterstock, from a series on road signs I made. I drew it in Illustrator and saved it as an EPS.

If you're an artist, create vectors--they sell well and if they are sufficiently detailed, some microstock sites will tag them as more expensive downloads--meaning more money to you. Selling vector images at Shutterstock means working within the (essentially) all-you-can-eat package--which can also mean many, many downloads.

Make sure to look into referral programs on the various sites. Many of them will offer rewards for users that you refer to the site either as photographers or stock image purchasers. You can send your referral link to interested people by email or include the link on your personal website. Shutterstock and Dreamstime are particularly good for rewarding successful referrals.

Microstock, like most things, disproves the old saying, "Build a better mousetrap and the world will beat a path to your door." (Ralph Waldo Emerson?). If you want to make a sale, you need to get the image in front of people's eyeballs, and to do that, you need to find as many relevant keywords as possible. The more searches that hit your keywords, the more that your image comes up in front of designers. (Note that it is important not to "spamdex" your image in an attempt to get your image viewed using keywords that don't match the picture.