So I actually do commentary during our monthly tournaments when we stream. Personally a couple things I keep in mind while commentating.
People are there to be entertained. If I'm not enjoying the match or get distracted it shows. So I try to make even slower matches or one sided matches entertaining.
Knowledge. I know a decent amount and I love to explain the thought process or the deeper tactic. Fighting for neutral, the options someone has with meter.
Hype. Sometimes you get some super tense situations. Even with those moments I try to remember that even though I'm getting nuts, it's still a job I have to do.
Dead air. I try to do my best between matches, or dead air. Nothing can kill a stream by dead air (and not during a regular break). Streamers tune in to watch to be entertained. The same way you would watch sports or any TV, I feel it's up to the commentators to keep the viewership, especially when there aren't any matches on.
Another thing I try to have anything I say contribute and add onto the video. To compliment it, and not take away or distract from it.
And of course, have fun!
Oh I also tend to circle back and every few matches or so to give the details of the tournament etc. People are always popping in and out of streams so I like to give the basics again for people who just tuned in.
Commentary is tough, I give so many props to any fellow commentators. For my favorites, Lord aris. Tasty Steve, yipes, ultra Chen. Tom Brady mk sometimes. Lord sp00ky. Rush hour. Just to name a few.