Cold sores are caused by the herpes virus (type 1 non-STD variant), so cures for herpes are really cures for cold sores.

One trick with Herpes is concentrating on what amino acids you consume.  Herpes loves and needs arginine and hates lysine.  So for arginine avoid certain nuts (like almonds, brazil nuts, peanuts, walnuts, pumpkin/squash seeds, sesame seeds, red meat, and some cereals (like oatmeal).

Then the individual should make sure they take 1,000 to 1500 mg of lysine a day.

Other tricks for cold sores include lemon balm (a popular brand is herpilyn), nasalcrom, and buttermilk.

Because cold sores are related to viruses one suffering from them would benefit from checking out my general anti-viral page.