If you would particularly like something later in the flight or you cannot/willnot eat much of the menu then ask for something to be kept for you.
For example, if 2/3 of the menu has cheese on it and you loathe cheese (like me) you may be out of luck later. If you are an omnivorous trencherman, less planning is needed.
I would expect some food left part way through a flight, if you genuinely don't mind what it is.