I think a healthy diet + vitamin D supplements throughout the darker months definitely helps. I also take a zinc tablet or two if I'm starting to feel like I'm getting something and I feel like I recover more quickly. All my own experience though so whatever works for you.
All the above, plus cold water immersion (promotes brown fat/Mitochondria production) my mum sea swims everyday all year round and gets about one cold every 10yrs, she's almost 90.