Digigal, my friend is prone to cold-sores but after taking tablets of l-lysine from the chemist he rarely gets them! It is by no means a total cure for them but they very significantly reduce the rate of occurence!
You see...when the Herpes virus is replicating itself in your cells (eventually leading to a cold sore) it uses up an amino acid called arginine which is found in your cells. But if your cells contain a high level of the amino acid lysine, this lysine takes up space that could otherwise be filled by arginine. So, the higher the ratio of lysine to arginine in your cells, the lower the rate of replication of the Herpes virus and the lower the rate of occurence of cold sores! So, avoid arginine, consume lysine!
Now, the thing is that not that many foods in nature contain high levels of lysine; ones that do include fish, chicken and turkey, as do eggs and some dairy products.
But to be honest, you are better off obtaining tablets of lysine from Holland and Barrett or somewhere (normally I would have dimissed stuff from a shop like that as never going to work but I've seen that this actually does work!) I think my friend takes one a day.
Avoiding too much arginine is another thing you should try to do. Nuts seeds and beans provide high levels of arginine...and so does chocalate! I wouldn't go mad trying to cut these things out of your diet too much but its good to be aware that they have small causative affect in why you get cold sores.
Main thing though is to try the lysine capsules.