I fasten a 1x4 horizontally between the studs and use the hole in the back of the box whenever I can.
I hate having wiggly boxes after the drywall is up and painted.
There's also the bracket with the little feet, but the scrap wood is usually cheaper and quicker.
Pages 2 and 7 on:
http://www.erico.com/public/library/Fixing/FieldCat/LT0452.pdf
