I get stress-induced cold sores often. What can I do to prevent them?

Lysine. The the intake of the essential Amino Acid lysine may help you reduce the outbreak of cold sores. You can start with 500 mg tabs and possibly increase it to 1000 per day when you feel the cold sores coming out.
Diet changes. If the actual stresses cannot be avoided, then avoid trigger foods like sugar, caffeine, alcohol, chocolate and nuts. You can add Amino Acid lysine to diet as a supplmement. 1, 000 mg per day. Give or take.