Use different mat sizes. Creating differing visual spaces between frame and photo among all of your prints will give your gallery wall some depth. Try to have some prints take up their entire frame, while others rely on a thick mat to fit inside the frame.
Establish a focal point. Make the center of your display eye level, where most people's gaze will generally rest. The focal point can be a large frame or a midline for your frames to surround. There's no right layout for your gallery wall.
Pick photos and prints that enhance your room's colors. Test prints out against your walls before framing and hanging - you won't know what the starkness of a blank wall will do to your art.
Use frame hangers. It's likely that you'll want to replace pictures or move frames around. Adhesive frame hangers are the best way to safely achieve portability and avoid wall damage.