Cold sore outbreaks can actually be triggered by stress and fatigue. If you've been stressed out in your life recently, it would be a good idea to relax and do what you can to eliminate the stress in your life.
As a preventative measure, I recommend Carmex lip balm. It's available in most drug stores. I use it regularly and no longer have problems with cold sores. I make sure to apply it once or twice a day if things are particularly stressful in my life or I'm under the weather. I've also found it effective at keeping cold sores away when I feel the tingling of a cold sore starting. Sure, you end up with a bit of balm on your face, but it's a small price to pay for avoiding a cold sore altogether!
Treating during the tingling stage is most effective. While most people feel the tingling, it's been my experience that cold sores can also turn up following a dry skin sensation. If the cold sore has started to appear or developed fully, Abreva, while expensive, can clear sores up very quickly. I find it usually works within two days for me. I don't recommend Abreva as an early preventative measure just because it's so expensive -- $15 for a very small amount, whereas Carmex costs only $1.29 and you get five times as much.
Carmex really has been fantastic for me. I find that not only is it wonderful for keeping away cold sores, it also clears up any minor skin irritations I have. I have very sensitive skin and keep several tubes of Carmex around so I always have one close at hand.
If you find that you still develop cold sores or that your cold sores are severe and you get sick frequently, you may want to see a doctor, as frequent cold sores (and being sick frequently in general) can indicate a problem with your immune system.
Another possible trigger for cold sores not frequently mentioned is sudden change in climate. I used to get cold sores a lot when travelling between places that had vastly different temperatures and humidity. It was always either travelling or stress in my life that caused sores to appear. Since I started paying attention to these things and keeping Carmex on hand, I have been outbreak-free.
Hope this helps you resolve your problem!