I have never been able to keep pictures hanging straight on the wall. I'm obsessive enough that crooked pictures drive me up a wall. I don't know if I don't hammer the nail in far enough (or maybe I hammer it in too far?) or if the hangers on the pictures are crooked, but it just doesn't happen for me.
In my last apartment, I tried double-sided tape to help my pictures stay level. Unfortunately, the combination of too-sticky tape and cheap, flat paint, meant a lot of extra spackling and touch-ups when it was time to move out.
Why it never occurred to me to use the wonder that is poster tack is beyond me. I stick a pea-size piece of poster tack on the bottom edges of the frames and press them to the wall. The pictures in our house are (mostly) level and the paint remains intact. Win-win!
Poster tack works for me. Do you have any picture-hanging tricks?