Easy! Shift!

What you basically want to do is this (you'll need to use actual codes instead though - i don't have my player handy so this won't work unless you use actual button codes here)

<button>.S=<player_on>.S ;When shift lock is ON, the button turns the player on and turns shift off
<button>=<sleep>.S ; this turns the player off and then turns shift lock on

See this for the complete scoop...