I've had pretty good luck with the following (simpler) method:
- Look at your video analytics to see the ones that received the most views from recommended videos
- The tags in those videos are valuable (recycle them)
- If your username is unique tag it
- Find a word or phrase that describes your channel / videos and use it across all videos.
- a 'tagged playlist'
- a tag on YouTube can be something like a tag line ('just do it')
I only have a few videos in a niche I am hoping to expand into but this has proved useful and I can see spikes the day after I did it.
From some playing around I think that if someone finds your video by tag and then they bounce quickly, it weakens the power of your tag.
A while ago I read an article about how some of the 'AI' parts work and verification by acceleration.
Basically if Person A posts something and Person B finds it relevant the tag will be stronger than if only Person A had found it. (A + B)
Now let's say Person C comes in and also finds it relevant. (A + B + C)
You might think well that's going to take a long time to build relevance. But here's the magic.
Now we have A connected to B and A connected to C. But B and C have never shared relevance. As soon as B and C share relevance the tag grows stronger yet again(this forms a triangle where each person's reputation is the length of the line.) This time it bonds as a mutation & distribution! (A + B) + (B + C) + (A + C) After you keep adding person D,E,F,G it will continue to grow. (not as a triangle, the remaining Persons circle the triangle keeping an eye on the Persons bonding the triangle to further verify their relevance.)
The trick,
I believe, is connecting people. Yes you want an audience, and by you collecting that audience advertisers will know better where to target.
It's like if every time you flipped your light switch nothing happened. You turned your neighbors lights on. Only your neighbors are huge incorporations.
Another example is those fake facebook accounts. Chances are the person behind them didn't just add everyone they saw. They attack friends list, sheep in wolves clothing. The bot detectors and spam filters back off once you gain authenticity. How do you do that, saturate your unauthentic self with authentics.
I love this stuff, behavioral computing I think. It's beyond prediction really. Its a schematic... that works...
for now.
I think I should make a youtube video on this

at the very least a diagram.
Sorry for the ramble, hope this helped in some way.
It's kinda like how bitcoin works... Nobody knows then everyone knows.
EDIT: I'm going to get a few more old verified accounts and test tag building. I think if there is a video trending you can do more than just follow the trend. There is a way to take it to the next place. Not a video with 1,000,000 views. Basically find a high viewed video and place in its related purposefully to gain subs. People say subs don't count. Sure for your analytics its a good idea for google to make it 'not count' but of course subs can = views. Subs probably build more relevance than anything. Sub means I found what I'm looking for in a way.