I'm using a combination of Google Adwords, TubeBuddy, and YouTube itself.
I start out locating keywords that get 100-1000 searches a month. I'm trying not to go head-to-head with the big boys on the popular searches.
Then I punch those keywords into the TubeBuddy tag explorer and see where they rank.
Then I type them into the YouTube search bar and see if they pop up as a top 10 search term and where they are on that list. The sooner they pop up the better. For instance, if I'm typing in something like "employee rewards and recognition" and I see that in my suggested searches at the point of typing "employee rew". That's pretty decent. Especially, if it's listed at or near the top. If I get all the way to "employee rewards and" or worse "employee rewards and rec" before it shows up...not so good.
Here's an example of the keywords for one of my videos. The first number after the keyword is Google Adwords, the second number is TubeBuddy, and the last number is where it showed up in search suggestions (if applicable).
reward and recognition ideas 260 78 5
rewards and recognition ideas 320 78 10
employee rewards and recognition 480 73 1
team recognition ideas 260 73
staff recognition ideas 320 77 3
ways to recognize employees 210 75
how to recognize employees 210 71
employee recognition award ideas 140 78
So here's what I ended up using for my title (primary and secondary keyword) and my tags...
Employee Rewards and Recognition - How to Recognize Employees
employee rewards and recognition, how to recognize employees, ways to recognize employees, reward and recognition ideas, rewards and recognition ideas, employee recognition award ideas, team recognition ideas, staff recognition ideas
I like to use something like "how to do whatever" as my secondary keyword since that's what someone is likely to type into the search bar.
I hope that helps!