I don't know how I hung up large pictures before I got myself a Accu Mark Gecko Grip Level made by Black & Decker from Home Depot. This weekend, I saw it at home depot and picked it up. I then went home and hung up a large mirror in my entry way.

First I measured the hanging holes on the back of the mirror as illustrated in this picture:

Black & Decker Accu Mark Gecko Grip Level

Then I put the leveler at the preferred height and made sure it was level. I then measured to make sure it was centered in the wall. After that, I drew lines in the holes provided in the leveler, as shown here:

Black & Decker Accu Mark Gecko Grip Level

Then I screwed in the screws and anchors into the wall and hung it up.

I don't know how I managed to do this before I got this leveler. This could have saved me a lot of time. No more separate rulers, no more making mistakes, etc.