1. Decrease sugar intake, which depresses your immune system. Yes, fruit juice can actually increase your chances of getting sick.
2. Eat more veggies, they are the best source of the vitamins, minerals, and phytochemicals that are known to protect the body against many diseases, including cancer.
3. Increase vitamin D intake, which boosts your immune system, ideally through sun exposure and seafood (particuarly salmon and tuna).