When we moved back to DC from Hawaii, just like you I was getting a cold every 2 or 3 months, with the symptoms lasting for 2-3 weeks sometimes. Here is how I stopped the cold cycle in its tracks.
FIRST: Wash your hands, all the time. Every time you touch something someone else might have touched (doorknobs, handrails, public telephones (ha!), etc.) remember not to touch your face or especially your eyes until you have had a chance to wash your hands. Seriously, I probably wash my hands 10 or more times a day, especially at the office or (god forbid) after my commute on public transportation - after doing that I call my hands "Metro hands" and I can't imagine all the germs on them. When you go to a public bathroom, after washing your hands turn off the tap with your elbow or the back of your hand, and use a paper towel to open the door to leave. Since I started this regimen I get 1-2 colds per year, max.
SECOND: Once you even get the barest inkling that you're coming down with a cold, hit it with zinc cold supplements - HARD. I mean, the minute you get the suspicion that the tickle in your throat means you're gonna be sick, start IMMEDIATELY. The directions on the supplements I use here (ColdEeze, Zicam) tend to say one every four hours. In the beginning, I do it every two hours. I know the scientific jury is out on zinc and colds, but I am a true believer - the last cold I got lasted all of three days from start to finish and was really mild.