I am tired of installing a new program from the command line and having to use the rehash command so zsh can recognize the new executable in my PATH.

Adding the following line to your .zshrc will refresh it immediately after installing something from pacman or yaourt.

TRAPUSR1() { rehash}; precmd() { [[ $history[$[ HISTCMD -1 ]] == \
    *(pacman|yaourt)* ]] && killall -USR1 zsh }

The above code will also refresh for other terminals that are already open, so no need to close it or rehash it again. Change pacman|yaourt to your needs, say pip|apt-get.