Tip of the moment: Leave the holes in your walls alone.
In preparing a bedroom for our new baby, I needed to install a proper phone jack where the current one was simply hanging out of the wall on wires. In the process of fooling with it, I managed to make the hole bigger and bigger, so I had to replace a section of drywall before I could install the outlet.
[From my "Past Tips of the Moment and Past Thoughts" page. This was the seventh tip and 12th post overall. It refers to preparing what had been—up until that point—our guest bedroom/storeroom as a nursery for our first child. — 28 May 2009]