The obvious first step is to select the pictures, paintings or mirrors that you want in your gallery.
A good way to prevent experimental holes is to cut a paper pattern of each frame that you plan to hang. After you cut the paper place it on the back of the frame and feel for the hanging hardware. Make a little pin whole or marking of some sort to identify where the nail should go.
If you want a more unified look selecting the same type (color, manufacturer, wood, silver) of frame in different sizes and shapes conveys continuity yet is still fun and free flowing. photos courtesy of Pottery Barn
Here are a couple of other tricks using household items.
If you are hanging something heavy it is important to try and nail into the stud. If you don't have a stud finder tool you can use an electric razor. Simply turn it on and run the razor across the wall. You will notice a distinct difference in the sound the razor makes over hollow wall and the sound when it hits the stud.
To keep your pictures from shifting wind some masking tape around the center of the wire or place some masking tape on the back four corners of your picture and press against the wall.
An inexpensive way to get an expensive look it to buy unfinished frames for the hobby store and stain them with liquid shoe polish. Brown shoe polish gives the wood a walnut glow, oxblood emulates a mahogany or cherry color and tan polish will look like maple.
I hope these tricks will come in handy at your house!