Keep on snacking whatever you do, always keep something around. When you are full, you can drink mass gainer, milkshake or something like this instead of sodas, it's easier to stomach than solid foods. Choose more caloric foods and use more fats in those - ones that fit certain meals, don't just add random ones, it should be pleasure not duty. I personally like keeping potato chips and ice cream always around and just go through it when I play video games or watch something. These are really calorie-dense and help you add even couple of thousands cals everyday without even noticing you ate those.