The moment I started to avoid touching my face, nose, mouth, eyes and ears with my hands I started to have much fewer flu/cold infections. I always use a tissue if I wanted to. As a person who works for an airline and meeting a lot of passengers is a daily routine, this was a big problem for me when I started the job and I find this prevention method to be working perfectly for me.
I know that you see this written everywhere, wash your hands properly, etc.. but it really works. Most infections come from touching something and then "introducing it" to your body by touching your eyes or mouth or so on.
Also, just as an extra step, buy a mask and wear it. I see some passengers doing that from time to time. Keeping a hand anti-bacterial gel is also recommended, just don't over do it.
I personally do not believe in instant immunity boosters (vitamic C and the like), as far as I know it's not proven to work. The best way is to prevent the virus/bacteria from entering the body, not to let it in then depend on a "boosted" immunity system to fight it.
Extra steps specific to aircrafts:
- Wipe the tray table with something that contains alcohol.
- Armrests too
- If you're sitting in a window seat, wipe the window as well. Kids love to stick their runny noses there!
- After using the toilet, use a tissue to open the door and when closing it. The handle will be a good place for all kinds of bacteria.
Again, these simple methods will provide enough protection. Do not use any advertised immunity booster medicines, keeping the viruses away is the way!