Tell some stories. Whenever I watch commentaries, I love it when the commentator tells a story about his life and what happened.
Make sure you don't use the same word too much.
Keep it cool, don't get too nervous.
If you're doing a walk-through, try to stay on topic on the important parts of the video.
Have a nice intro. Not a clip before your commentary, an intro to when you start talking. Kinda something like "What's up everybody, Chronos here, blah, blah, blah..." or something like that.
Watch some other commentaries that you like and try to bring those aspects into your videos.
Ask your viewers what they would like to see.