I usually just say, "Извините", or if I have to repeat, "Извините, можно вас?.." (lit. "Sorry, can I have you?" but it's actually more along the lines of "can you spare me a moment?" or something like this). Some waiters don't like being addressed as "девушка" or "молодой человек" (though, as Margarita rightly says, it's not that uncommon to address them this way) so I think it makes sense to stick with the short polite options.