Why yet another howto? There are really good ones on the net, like the git guide which is perfect to begin. It has good links including the git book to which one can contribute (hosted on git hub) and which is perfect for this collective task.
On stackoverflow, I would really prefer to see your favorite tricks !
Mine, which I discovered only lately, is git stash, explained here, which enables you to save your current job and go to another branch
EDIT: as the previous post, if you really prefer stackoverlow format with posts as a wiki I will delete this answer
