9. Wash hands several times a day
Admittedly, thorough hand washing does not directly strengthen the immune system. But it prevents cold viruses from getting into your body and spreading. Those who want to avoid colds in the cool season and who already have a weak immune system are well advised to wash their hands several times a day.
And honestly, think about what you're touching outside all day. When shopping, when traveling by public transport, in the park and so on. So, keep good hand hygiene and protect yourself and your loved ones.