While ignoring commands preceded by a space character is absolutely one of my very favorite shell features, it usually isn't enabled by default (YMMV).

HISTCONTROL=ignorespace # bash

setopt hist_ignore_space # zsh

These go into ~/.bashrc or ~/.zshrc, respectively. zsh is really smart about this and always enables you to retrieve the immediate previous line with arrow-up, no matter whether it's got preceding whitespace or not.