Probably have been devoting most of my energy into professional looking thumbnails than video editing actually, Even made some process videos.
Usually I'll find a background with some type of texture on it and saturate it for every new video. I'll Photoshop an image from/related to the video with a white outline around it and a drop shadow behind the image. Angled text seems to work really well too!
Small piece of advice from me for your channel art: be aware of the trends! Trends are moving quite fast, you should change your art once or twice a year. Keep it trendy and people will be more incline to click