If you have problems hanging photo frames as straight as you'd like, try dabbing toothpaste on the end of the picture hook. Household weblog The Industrial Cottage suggests holding your picture where you'd like it to go without touching the wall until you find the perfect spot, then gently tap the frame against the wall.
This should leave a small toothpaste mark to indicate where you should insert a nail. Wipe away any leftover toothpaste once the nail is in.
Today was a very interesting day [The Industrial Cottage]