There was this treatment called Cymex which I used for a while. But I've always found that Zivorax is best - I have tried own brand stuff as it's essentially the same ingredient but somehow it didn't make it better as quick so I switched back.
There's a herbal remedy you can use also - bit weird, smells like soil but great; it's called Watermelon Frost and you can buy it in Chinese supermarkets. Don't ask me what goes in there but spray it on when the first signs appear and it literally goes away in a day. I usually keep spraying it until I'm completely sure it's gone though. My friend whose Chinese recommended it to me and I've never looked back. I like how it dries out the blisters whereas all the other creams and stuff keep to just keep it moist.
Anyway, tried to control my stress so it doesn't happen as often and touch wood, it's still working.
next time you feel one coming (like when the skin tingles or starts turning red) try dabbing it with a witch blemish stick (they cost like £3) and it should be gone within a day ..i haven't had cold sores in years by doing that
..and remember not to share cups or kiss anyone.. and wash your hands if you touch the sores, so that they don't spread to your eyes or other places you might be touching..