I know that mine could use a lot of work, but I re-did them the other day, and they are a lot better now. As an amateur, I'm content with them, and if potential growth requires it down the line, I'l go harder on them then.
I'd say create thumbnails that stands out, use your gut-feeling.
Eyecandy, contrast, design, placement is all part of that.
Make it relevant to the feeling in the video so the viewer doesn't feel ripped off.