How do you say thank you for the food in Japanese?

Generally speaking, after meals a Japanese person will say some form of ごちそうさまでした (gochisousama deshita) which can be translated as "Thank you for the meal", but can also mean "That was a delicious meal"/"I really enjoyed the meal". Before a meal, ごちそうになります　(gochisou ni narimasu) can be said, to mean "Will be a treat" or "will be delicious", however it is more typical to say いただきます (itadakimasu) "Let's eat"*.

*いただく (itadaku) can be used as a humble form of the verb 食べる　(taberu) which means "to eat".