I am very sensitive to cold weather. Years of experience in many different places, hot and cold, and experience observing and talking with others, has led me to the conclusion that you can prevent common cold and flu almost completely if you stay warm. If you are wearing no winter clothes (insulation) and are dressed in a t-shirt and shorts/skirt, you will catch a cold if exposed for about 60 minutes when the outside air temperature is 20C (this is approximate; people who are very sensitive to cold will get sick at 22C, while the tough ones will get sick below 17C). You will also NEVER catch a cold with the same dress if the outside air temperature is 25C or above. Ambient air temperature and body insulation are the important factors for catching a cold/flu.
Main observation: In warm countries ( >30C temperature) or in summer (>30C) you never catch a cold/flu. This gives you an idea of how the cold virus works; it completely dies in hot weather-or does not multiply, the virus doesn't seem to incubate at all at high temperatures. This is the most vital piece of information you need to combat flu. I have flown from cold weather with a terrible flu to a warm country, and the flu was gone in 24 hours after arrival into the warm country. By warm I mean really warm, >30C temperature.
Staying warm is the surest way to avoid getting the cold/flu. As the air temperature drops below 20C, you need more and more winter clothing (insulation) and heat the ambient air.
Even if you are bundled up in all your winter clothes if you are breathing cold air, you may get catch a cold. Inhalation and exhalation of cold air is like convection loss of heat-it takes away a lot of heat from your body by the sheer amount of air you inhale and exhale every minute. The solution is to at least moderately heat up the air around you. This is the reason when people go climbing high mountains they heat up their tents at night-because even though they have the best winter clothes on, it is not sufficient, and the air with is breathed needs to be warm as well.
A good way to get some heat if it is not possible to heat up the room is to drink hot beverages-hot tea, hot coffee, etc. In the very cold Argentine Patagonia people keep drinking hot mate every few hours, which keeps them warm. Ingesting a hot liquid gives you a lot of heat in calories, and is a very efficient and cheap way to stay warm if heating the surroundings is not possible. Drinking hot liquids helps in avoiding the flu at the first place; and if you do get the flu, it will help it cure faster.
A common way people will catch a cold, without realizing it, is from excessive exposure to cold water-e.g. by washing dishes, or by drinking ice cold water. This can happen in the summer or the winter-and avoiding contact with cold water will reduce your chances of catching a cold/flu.
Common cold and flu are not contagious. If you are warm, it doesn't matter if everyone around you has the cold. It is like vaccination-if you are vaccinated, you don't care if other people around you are vaccinated or not. This has huge ramifications for children, who sometimes are forced to miss school because of the bad conclusion that the cold virus (or microbe) is contagious. I have a good deal of experience to believe that it is not contagious at all; and if you keep warm, you will not contract the common cold/flu. The recommendation to wash your hands, not come in contact with people who have flu etc. are useless - I have reason to believe that the virus is always inside our own bodies but in a dormant state, but incubates rapidly when the temperature drops.
If you are in a temperature below 20C and are not dressed to keep warm, you will catch a cold, even if you are in the summer. This happens to people all the time when the air conditioning (A/C) is too high. The solution there is to always carry some sweater or winter clothing in buildings which run the A/C too high. It seems to be that the flu/cold virus is always around us or even inside our bodies-but needs cooler temperatures to show it's effect. It is not necessarily contracted form other people having the cold/flu.
I have deliberately not gone into the details of what is flu vs what is the common cold, and what are the exact viruses (or bacteria-because some symptoms like phlegm are characteristics more of bacterial infections) which cause these. Medics don't even agree on this. I just tell you this from my personal experience-and I have gone for years without contracting the flu/cold. Even if you will come down with a cold after all this, it will be a mild episode, lasting 1 to 3 days. The symptoms and the suffering is considerably reduced if you remember to always stay warm even after contracting the flu/cold.
Lastly, lemons, gingers, etc. etc. are remedies which are proposed as solutions for alleviating the symptoms of the flu. My experience is that none of these work. The best solution seems to be temperature control, firstly to prevent you from falling ill with the cold/flu, and also to cure it fast.
Taking a pain killer like ibuprofen and sore throat reliever does help in reducing the severity of the symptoms.
Yearly flu shots are bad science, they do not work. You will catch a cold even if you get a flu shot if you are exposed to cold as outlined above. Here are details on why flu shots don't work. Avoid getting yearly flu shots.