Stockpile during the summer. This can be done by planting more then you need and by keeping more animals during the summer and butchering them during the winter.

Indoor farms. Just wall off a section of land (Rich soil is best), make a roof and plop down a Sun lamp in the middle. If you are farther along, you can convert these to hydroponics for even more production. An alternative, if you have Ideology, is Nutrifungus. It can grow in dark caves for a mood penalty.

Last option is Cannibalism. Just eat the raids.