It really depends on what style you want, but here are some general tips:
- Develop a template and stick with it (for a while). This makes you thumbnails recognizable for your audience.
- Keep it SIMPLE! Do not add too many elements and text to it, since the image will be really small.
- Test your thumbnail by shrinking it in your editing program or zoom out on it. Is the text readable? Are all elements clear?
- Don't put your full title on the thumbnail. It takes up space and doesn't add anything.
- Instead, add to your title with buzz words. Are you doing a make up video on bright colored mascaras? Your title can be something like "Trying out crazy mascara colors!", while the text on your thumbnail maybe says "Trying BLUE mascara!"
- Use either bright colors or stick to black and white. Nothing is quite as effective/readable as white text with a black outline. Do outline whatever text you use, for readability. Don't worry to much about the subliminal messaging of colors, it's more important which colors catch the eye.
- For personality based channels (travel, gaming, beauty) it has proven more effective to put your face in the thumbnail.
- Touch up your thumbnail screenshots. Blur backgrounds, enhance contrast, up vibrancy. All this will help your thumbnail POP.
Hope all this helps
