First and foremost, watch your weight. Many times, these treatments can cause excessive weight loss, so it's important to maintain a healthy weight so you're at your best and your immune system can stay up to par as much as it can. For foods to help you maintain your healthy weight, look to nuts for a source of good fats and high proteins. Fiber will help you stay full and will help you keep the nutrients in your body. Different concoctions of milkshakes also help keep down a nutrient rich product. Also look at vegetable blends and dairy products - good sources of nutrients and proteins.
Also be aware that when you're having these treatments, your immune system is going to be down. So we look to using a diet called a nutropenic diet or "no raw foods". These foods must always be cooked because raw foods may have micro organisms or bacteria that could make you sick. If you're having trouble gaining that weight or sticking with that nutropenic diet, try nutritional supplements. There are good products out there that provide all the good fats, proteins, and carbohydrates that your body needs; all in a small, convenient package.
Sometimes you're not going to want to eat and you're going to feel very tired. You may even be lacking the ability to taste. Try to country this with simple actions, like biting into a lemon right before you eat. This will stimulate your tastebuds and open you to different flavors you may not have tested before biting that lemon.