Pick one subject and one message. Use a short phrase only if the image cannot carry it alone, and keep text away from edges where platforms may crop. Strong contrast helps, but the thumbnail should still represent the actual video.
Preview the exported image at the small size viewers will see, not just in the editor. Check faces, fine lines, and lettering on a phone. If the image becomes cluttered, remove secondary details rather than shrinking everything. Compare the final export with the video title so they complement rather than repeat each other.
A clear promise is better than a misleading clickbait image.
Akousa