One of the side effects that everybody expects from chemotherapy is nausea and vomiting. I've been fortunate that in 2-1/2 years and three rounds of treatment I have never vomited from chemotherapy. I have had to deal with nausea ranging from a slight loss of appetite to "ugh I feel sick and can't bear to think of food".
Some of the advice that I was given was to try to stimulate an appetite by preparing my favorite foods. I've learned that that can be a bad idea. My first round of chemotherapy involved 6-hour infusion sessions, so I needed to eat something before and during the infusion. I chose some of my favorite snacks and meals: donuts, bagels, grape juice, applesauce, ham sandwiches. Unfortunately after a few weeks my body began to associate those foods with the sickness that I felt from the infusion. Whenever I saw or thought about them I suffered from associative nausea.
Some of those foods I have managed to disassociate from chemotherapy, but grape juice and applesauce still turn my stomach. I also have strong associations with smells: roasting coffee beans and certain brands of pizza remind me of snacks I ate on the days after chemotherapy.
My advice is to choose foods that you moderately like but not your favorites. For me, this means that if I must eat during chemotherapy I stick to snacks like pretzels, animal crackers, and apple juice. I totally avoid my very favorite foods (sushi, steak, certain desserts) on days when I'm feeling sick from chemotherapy. My current chemotherapy regimen takes only an hour to infuse, so I don't eat anything during that time.
One of the lessons that many people learn from fighting cancer is to enjoy all the little pleasures in life. Eating good food is high on that list, and the threat of weight loss from chemotherapy often means a license to eat as much as you want. Good nutrition is also important for staying healthy, so you should consume wholesome and high-quality foods. When I'm free of nausea, it's nice to be able to indulge in fine meals and desserts. Cost can be a concern, but I economize by getting restaurant meals as takeout (saving on beverages, side dishes, and tips) or learning to cook more myself.
I have also benefited from a couple of medications to alleviate nausea and stimulate appetite on those marginal chemotherapy days: Emend for the severe nausea from Camptosar and Marinol for the mild nausea from Xeloda.