1. Drink enough water!
Just because you may not be sweating like you do in the Summer doesn't mean that it's less important to get your daily amount of water. 8 glasses a day all year round... Tea counts as well, so add some peppermint, ginger or cinnamon to your hot water and drink up!
2. Get out and exercise!
It may be dark and cold whenever you have the time to exercise but you can take a nice walk during your lunch break and get some much needed vitamin D.
3. Get the right amount of sleep for you!
The Chinese believe that humans should go to bed when the sun goes down and wake up when the sign rises. This may be a bit unrealistic for those of us living and working in the modern western world but it's still important to get enough sleep. 6-8 hours of quality sleep will help keep you energized and less prone to illness.
4. Wash your hands regularly!
Bacteria and viruses are everywhere, make sure that you aren't carrying them around with you wherever you go :)
5. Keep stress down to a minimum!
High stress levels can make you more prone to illness. Try and work regular hours, do something that you enjoy at least one night a week and book a massage or acupuncture session to guarantee a stress free hour or two during your week!