Contrary to what people are saying, you don't have to increase and cut out anything. If you eat a balanced diet already (which includes chips, cake etc), then all you need to do is make sure you have the right amount of food on your plate, and eat often enough.
You shouldn't go more than 3 hours without eating, and the routine should be breakfast, snack, lunch, snack, dinner, snack ("snack" being a banana and a Flake, or a yogurt and a fruit etc). these meals can obv. be interchanged based on schedule etc. You should never skip one of these meals because you don't feel hungry, you should always eat them anyway.
Try and cut your plate size to 10". If you have plates that have a thin edge, you're more likely to take more than if you have plates with a broad rim (the illusion of space). Try to avoid buffets, people generally eat 2-3 times the amount they normally would at buffets. If you feel hungry, try to drink something before scoffing, thirst is quite often mistaken for hunger. If you find that lunch isn't keeping you full, try changeing your sandwitch for soup. Soup is proven to keep you fuller for longer than solid foods.
Also, fish is good, it's not very fatty but is pretty cheap and filling, for example if you make a chicken curry, swap the chicken for fish, which automatically brings the fat content down by about 1/3.
This is what the hospital has told me, and it works even though it seems like you're eating a lot!