Beware of sunlight and heat, both of which can damage art. When you have pieces framed, ask for UV-coated glass. And take note of windows and radiators before you choose a spot for anything precious.
Tighten slack wires on the backs of frames, especially if your goal is precision hanging (lining up art in a grid, say). A loose wire makes it hard to control the height of a piece.
To protect walls, use peel-and-stick clear rubber bumpers on the back corners of frames. Rubber pads, $4.50 for 18, dickblick.com.
Choose the right hooks for the weight of your art. Ook brand hooks offer a range of up to 160 pounds and leave tiny holes that are easy to repair if you change your mind. Shields professional hangers, from $5, ooks.com.
Fix mistakes on a white wall with Spackle (sold at hardware stores) and a fingertip. Apply a tiny dot over a hole, then smooth with a damp paper towel. (White toothpaste can do the job in a pinch.)