We started out with thumbnails that were clips from the video with text overlays and maybe some branding if we were featuring something. After a couple of weeks, I designed a template for us that included 1/3 white space for text, some catchy contrasting colors to break that third off, and a smaller but more focused clip from the video in the other 2/3. We have been using that template consistently and it has become very easy for our viewers to pick our videos out of a search result because it looks clean and you can tell it is ours.