Cold showers have helped me. I've already had a reasonably good immune system though. I am on the weird side and probably take 2-5 cold showers a day though.

If I feel a little off, I'll take Vitamin C and Lysine as early preventive measures. The rest of my healthy habits are quite subpar. I exercise frequently, but I eat garbage and I sleep even worse. Averaging about once a year in being sick?