Treatments for cold sores:
Aciclovir (or acyclovir) cream - This is available without a prescription in the UK. The main brand name is Zovirax, but there are also generic versions which are cheaper. Application of the cream at the "tingle" stage can stop the sore developing. At worst, the sore clears up much quicker. Talk to your pharmacist. http://www.zoviraxcream.com/pdf/ZovrxCream.pdf
Compeed, who make foot blister treatments, have launched a patch to apply to cold sores on the lips. It seals the area, makes the sore less unsightly, and is claimed to speed the healing process. It doesn't have any active ingredients, and you can't use cream with it because it wouldn't stick to the lip. I don't know if it is available in the US.