I get one or two colds a year. It's pretty simple really; get out of the habit of rubbing your eyes and picking your nose as this is how viruses get into your upper respiratory tract and infect the mucosa.
You can also reduce the chance of getting colds by ensuring that everybody washes their hands as soon as they enter the house from outside; this will reduce the numbers of viruses on door handles etc that everyone touches.
The story about vitamin C is bollocks, it arose because the body temporarily loses the ability to absorb vitamin c during a viral infection. However there is some medical evidence that zinc increases the mucous membrane's resistance to infection.
Lastly buy a bottle or Corsodyl mouthwash and as soon as you feel that tickle at the back of the throat begin gargling three times a day with it; it is extremely effective against throat infections as well as oral problems like ulcers, gum disease, etc.