In art school I used the string and pushpin method:
Measure up 57" from the floor where you want your first image, then do the same where you want the last one. Sink a pushpin into each spot and then run some string or thread between the two. This will give you a level line (as long as your floor is level, which it isn't always) to hang along so that a line of images will be in a perfectly neat row.
The hang and level boasts one of our favorite features, eliminates “mistake” holes and scratches. It also helps to hang things level (if you couldn't tell from the name). Reader Studio Starter reminded us about it in our post about a hanging that demands precision.
Here are some other tools that are helpful for hanging artwork:
How To: Arrange and Hang Picture Frames Using Butcher Paper
Tools: The Hang and Level
AT Reader Tip: Hanging a Frame with Two Holes The Easy Way
Art Gallery Layouts
Right tool for the job: Laser Level
One Finger Installation with Push Hang Drywall Hooks
If you're a renter that isn't allowed to hang things on the wall, how have you gotten around it? Do you use any of the tacky sticky stuff to hang things? Or wires? Or do you just patch the holes later?