Ctrl-p in emacs takes you to the previous line. Since readline uses emcs-style notation by default, Ctrl-p takes you to the previous command in history (and Ctrl-n to the next). I don't know about zsh though.

Also, other emacs keys work like back: ctrl-b, meta-b, forward: ctrl-f, meta-f, start of line: ctrl-a, end of line: ctrl-e etc.