Thumbnails are often seen very small... so the main things are to ensure the text or image is clear and easy to read/understand at even a very small size. Don't do anything overly complex with tiny parts you have to see to understand, as it will get lost. But also ensure it's high res enough and interesting at full size. Also bear in mind YouTube puts the time over the bottom right corner, so don't put anything vital in that area ^_^
I would say also; take a screenshot of a results page for your search terms. Put your thumbnail onto that page in Photoshop/whatever and zoom out... see if you can spot yours, does it stand out? Does yours looks like one of the better ones? Would you click it?
There is also merit to have a 'look' to your thumbnails if it's a series of related videos. Different enough to not be confused for each other, but strong enough brand recognition that loyal viewers will see it's another by you and click through in preference to a competitor.
Another tip to those who want to use a still from the video in the thumbnail... take a few posed photos when you do the video. They will always be clearer and better than stills grabbed from the video. Use a timer or remote if you need to.
I hope this helps! ^_^