I have done a fair bit of home remodeling, and I have had a few instances where I wanted to put an outlet or switch located closely to something else.  I threw this template together a few years ago to make locating the boxes with a cover plate easier.  Locate where you want your cover plate or box to end up, mark the outline of the box, and cut out the drywall to fit an old work box in it's place.  I use an oscillating tool for drywall cutting and it leaves clean cut lines.

I originally posted this to Cults a few years ago, but has to recreate it at the request of someone from the original site I posted it to.