Well for example your One Channel trailer, the video you use is usually one that you're trying to make a gateway video. Catch people within the first 10 seconds, don't use long boring intros.
If I can give an example
/watch?v=DnqC-ONuNSQ (It won't allow me to post links, however this Youtuber manages to make almost a perfect gateway video)
In this video he manages to capture peoples attention very well on some points, and others are lacking which is why the video has only 40k views.
1st of all: The content/music in this video fits together perfectly, and it's from many different games, giving him a big wide audience.
2nd of all: He manages to capture people within the first 10 second with either something funny, or just unexpected.
Now the things he could have done better
He could have given the video a better title/thumbnail/tags this would have improved it as a gateway video, making more people click it
If you don't have gateway video(s(, getting 1000 or even 10.000 subs is going to take years, take for example my channel, I found out that my Darius video with 100k+ views turned into a gateway video, so I made a video series following on the one. Now all 10 videos so far act as gateway videos, and yesterday I beat my record with a stunning 714 subscribers gained in one day.