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.
I feel like simplicity is the best way to go, I recently started a new YouTube channel called Apollo's JukeBox (the video/thumbnail is in my signature below) where I just overlay the word "Apollo" over a relevant frame of the video. Just make sure the picture tells a story or at the least has interesting aspects.
hmmm, i like your thumbnails! the blurred background is what i like, it really allows your front title to stand out, do you use photoshop? you could add a lense flare and maybe play around with the camera raw filter for your backgrounds and maybe add a little outer glow to your titles?