integer toggle; //simple integer switch -- has two possible values, 0 (false) and 1 (true)
default
{
    state_entry()
    {
        toggle = FALSE;//set toggle to false (by default it is anyway)
        llListen(5,"",llGetOwner(),""); //listen on channel 5 to messages from the owner
        llParticleSystem([]);//start with particles off
    }

    listen(integer channel, string name, key id, string message)
    {
        message = llToLower(message);//convert message to lower case, so we aren't case-sensitive
        if (toggle == TRUE && "off" == message || toggle == FALSE && "on" == message){//if either I'm emitting particles and get the "off" message or not emitting and get the "on" message
            if ("on" == message){
                //do stuff to turn particles on
            }
            else if ("off"==message){
                //do stuff to turn particles off
            }

            toggle =! toggle;//switch the value of toggle

        }
    }

}