If you've ever used picture-hanging wire in your life, you know it's prone to scratching up walls and being a bit of a pain to use. To alleviate this problem, This Old House recommends using dental floss instead.
Typically, the back of a frame is crossed with a piece of braided wire. This unbraids over time and can scratch up a painted wall if you're not careful. Dental floss is soft, so you don't have to worry about scratching the wall, and it's easier to work with since you don't need wire cutters. If you're hanging a heavy frame you can always braid the dental floss together so it holds. Find a few more unusual tips for using dental floss over at This Old House.