Taking Vitamin C helps in this as cold sores or the herpes virus waits until our immune system is compromised to attack us. Abreva is one of the best over the counter things I have found to get rid of the sores quickly once they pop up. Camphphenique or camphor will dry them up but it is slow.
Hand washing and not using towels or wash cloths that may have been touched by anyone who has a cold sore should be added to the above post too.