Hat-tip to Lifehacker for showing me this method.

Put a tiny dab of toothpaste where the screws should go, then press the item to your mounting surface. Instant screw guides!One way to make a work space look nicer and reduce clutter is to use available mounting brackets on items to attach them to walls, desks, or other surfaces that aren’t the floor.  Sometimes getting the correct screw positioning for such installations can be difficult, but you can make the process much easier and faster using something you’ve already got handy in your home: toothpaste.

After choosing a mounting location, simply put a very small dab of toothpaste on the item to be mounted at (or very near) each point that will attach to a screw head.  Then touch the item to the mounting surface in just the location that you would like it to be.  When you pull the item away from your mounting surface, a small dot of toothpaste will show you exactly where each screw should be placed.

After installing the screws as necessary, simply wipe any excess toothpaste off of the item and the mounting surface, and your item should mount just perfectly.