Do your best to maintain good mental health. When you're stressed or anxious, your body can't properly fight off illness. Use relaxation techniques and other stress-management strategies to help keep your mind in a positive place. Seasonal depression can affect the immune system, too, so be sure you're getting enough sunlight. Get outdoors on sunny days and talk to your doctor about treatments for Seasonal Affective Disorder (SAD).

Natural Remedies

Echinacea is the most popular natural immune-system enhancer available. Many people swear by it, and wouldn't go a winter without it.

Probiotics help maintain a healthy stomach, but they also contribute to the overall health of the body's immune defense. You can take probiotics year-round to keep your body working in top condition.

Occillococcinum is a difficult to pronounce but easy to use homeopathic treatment for flu. It's said to be effective against flu symptoms when taken at the first signs of illness. It has performed well in clinical trials and is available at many national drugstore chains.

There are many natural supplement products that have (or claim to have) immune-boosting potential. Always do some homework before shopping for these products so you get one that's right for you.