How can I stop getting sick?

Recently moved to the UK and can't seem to stop getting sick. On my third cold in 3 months and I'm over it! Any tips (and preferred medicine brands) greatly appreciated!!!

Update: wow, was not expecting this response! Making a list of all the supplements and heading to Boots!! I should have noted in the original post I'm doing all the common sense things (hand washing, sanitiser, not a smoker, drink litres of water a day etc). Thanks again!!