The best way is too start eating a little bit more then you usually do. To the point where you feel uncomfortable. Your stomach will stretch and you will be able to eat more eventually. When I was in the Air Force during basic training, you had to shove a bunch of food in you as quickly as you can, plus three drinks per meal. At first the three drinks alone filled me up and I couldn't eat much. But since the demand for nutrients to fuel my body was high, within the first week I was able to finally eat more. By the end of training I could eat the whole time without getting full, and that's alot of food. Now that I'm not doing that anymore, my appetite is back to normal.