On adding a missing face to a picture in iPhoto the default behavior of the face box when resizing is a centered resizing, which is far from easy to get custom to. If you press the Option key when resizing, you get the more common from-corner approach.
[robg adds: I couldn't find this in the list of iPhoto '09 keyboard shortcuts. I'd check to see if this is in the manual, but ... iLife '09 doesn't seem to have manuals available of any sort, just video tutorials. I'm assuming this trick is probably covered in one of the videos, but I'm not going to sit through one or more just to find out.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090208054745437