Nail your nutrition and hydration, both play a key roles in supporting your immune system. Make sure you have a wide variety of natural, whole & unprocessed foods with the appropriate blend of fat, protein & carbohydrate.
Dress appropriately when you're out training in wet & cold conditions.
Get 8 hours sleep a night.
Reduce your lifestyle stress. All stress takes a toll on immune function so you do need to manage your stress load. If you have higher than normal amounts of stress in an area of your life you'll need to reduce stress in another to ensure balance.
Manage your training load. Over-reaching is a sure fire way to bring on illness.
Wash your hands especially before eating.
Avoid being close to people who are ill.
Try not to touch your face or put your fingers in your mouth or nose.
Don’t share food or drinks with friends.