This is very non technical... but...
Put lots of love into it. I had no real system.
I think every photo tells a different story and means something to different people.
Therefore each photo needs to be tagged differently.
MC is just a tool that helps you do that, in the same way
Photoshop is a tool for correcting small issues with the image itself.
I tagged all the people in the photos, and where they were taken,
then gave them captions if relevant.
Make a backup when you're finished!
I used the tag window a fair amount, and also the panes, tagging mode.
I think playing helps the most!