I'd like to think my past few thumbnails have been pretty decent as I get better with every video I make. I use Photoshop CS6 to make them and once you get acquainted with the software it becomes pretty caked. I'd say if you have writing on your thumbnail, make sure the font is pretty ( not just a normal default font ), i'd hugely recommend dafont.com to find a good one. Then for images it really comes from looking around seeing what big youtubers in your category thumbnails look like and reflecting that upon your thumbnail. This doesn't mean copy it, but use it as a sort of "let me make mine look as clean as this" if you know what I mean. Hopefully that helped.