Whether you use an image or a color as your background, use a contrasting color for your text to ensure it is clearly legible.
I recommend using San Serif fonts for their bold simplicity.
Adding an image of the video's subject will draw your audience's attention.
Less is more. Thumbnails with too much going on will likely turn viewers off rather than attract them. The same goes for text. It isn't necessary to cram your entire title into the thumbnail. Instead, use a few key words.
Although not necessary, putting your face on the thumbnail improves your brand identity. Your viewers are more likely to recognize your face and be drawn to your videos over others.
