I found out about “git add -p” today. Too often my git commits are too large, incorporating more than one change. git add -p (patch mode) lets you add some of the changes in a file and not others. John Kary has an excellent set of videos on git add -p.
http://johnkary.net/blog/git-add-p-the-most-powerful-git-feature-youre-not-using-yet/
I wish I knew about this a long time ago.