Catchy clickbait thumbnail pics help too I think. How many times have I clicked on a yt video because the thumbnail pic looked interesting. Only to be disappointed by the actual video. But its the click that counts. I think you have to be up to a certain level of subscribers and verified before you can pick your own thumbnail though. Commenting on other 'tubers videos gets your channel more visibility too and can draw viewers in out of curiosity. Plus getting a "shoutout" from another well established youtuber can really draw the traffic.