Great question. First off, I always abide by the rule that you should never have text of a bright color overlaying a background of a bright color. Or a text of a dark color overlaying a background of a dark color. Secondly, the font style of your text should always be legible to viewers. Thirdly, the font size of your text should be of a decent size of course. And lastly always consider what your thumbnail image will look like at a small size and create your thumbnail always keeping that in mind.