Eat omega-3s every day. The importance and benefits of omega-3 fats, from fish oil specifically, cannot be overstated. In fact, it's a good idea to shoot for 500 to 1000 mg of Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) in combination per day plus 12 oz. of fatty fish like wild-caught salmon, trout, tuna, sardines, anchovies and halibut per week. If you don't eat enough fatty fish, increase supplementation to 1000 to 1500 mg of DHA and EPA (in combo).
For decreased inflammation and improved immune function, it's a good idea to increase fish oil and other omega-3 fats while decreasing processed foods as much as possible to re-align your omega-3 to omega-6 ratios. It's the imbalance of this ratio from low-fish, highly processed diets that is correlated with numerous diseases, failed health and poor immunity.
Take your supplements: Multivitamins, Vitamins C and D and iron if needed. As an athlete, you have higher nutrition needs than more sedentary people. Unfortunately, you may not always be able to meet your nutritional needs through food. To stay healthy and support your body best, include a high-quality food-based daily multivitamin. I usually recommend the brand Rainbow Light, which includes freeze-dried probiotics in the multivitamin. Then, be sure to consume 500 mg of Vitamin C and at least 1000 IU of Vitamin D per day.
Also consider taking iron, especially if you're female or a vegetarian athlete. There's a consistent correlation between low iron status and chronic colds, flus and even mononucleosis.
Get enough sleep. A common denominator between adult athletes? Huge demands on time. It's easy to allow your responsibilities to crowd your day and infringe on your sleep. But, research supports that too little sleep can increase your risk of illness. It also undermines recovery, stress-relief, fat loss and overall wellness. Bottom line: make sleep a priority.
Know when to train and when to take a break. When you first begin to feel sick, it's a good idea to cut your workouts short. Shorter workouts can actually boost your immune function, whereas long endurance training can make it more difficult to fight off a cold. It's usually a bad idea to "tough it out" and push through a long workout when you're feeling lousy, so keep it to 60 minutes or less until you've recovered.