If i'm getting cold sores more often, then i'll take a course of Lysine tablets.
When you feel one coming up and you are at home you can apply toothpaste to it, but it has to be actual paste, not the gel type toothpastes. The toothpaste cools it right down and takes the sting away and seems to prevent it getting bigger. It also dries on the cold sore so that you can go to sleep at night and not worry about it smearing onto your pillows or bed linen.
An ointment i love is Carmex as it's good for cold sores or for dry or cracked lips and it's a clear ointment so you don't have to go around town with some white glob sitting on top of your cold sore and drawing even more attention to yourself like some of the other creams do.