I often ask for "take-home boxes" (you'll get funny and confused looks for a "doggy bag") if there's a significant portion of my meal left. But I find that portion sizes are more reasonable here than in the US, so I don't need to ask for them as often as I would there. I've never, ever been refused one, and the waiters have occasionally encouraged me to add stuff like bread from the basket on the table as it'd be binned, too, and they also hate seeing waste.