I usually make up my lunch the night before with a sandwich consisting of grilled chicken, cheese, lettuce, tomato, cucumber, grilled capsicum, beetroot, onion and carrot. The sandwich alone fills me up, but I also take a handul of nuts, and a piece of fruit. I might also just bring a can of tuna to nibble on as a snack. I also make up a salad (i buy the coles salads in packs) to that I add some nuts, avocado, tomato, onion, low fat sundried tomato, and some balsamic vinegar as dressing.
The key it to be organised and plan your meal out.