I just go to the bathroom and ask for something on my way back to my seat...works every time and let's them know I am awake and would like to be checked on.
Did this on a LOT flight recently (WAW-ICN) and the FA just pointed to the bottles of water on the drinks trolley with a 'you can serve yourself look'. Same thing happened on the return leg. In fact, all the Y cabin crew were sitting together at the back, were they spent virtually the entire flight chatting with each other while the pax kept coming to serve themselves drinks.