Boost immune system. "canker sores" are a loose name for open mouth wounds. The recurring open sores are commonly herpes simplex viral lesions. They are reoccurring when your immune system is low due to stress, poor dietary habits, sleep loss, other chronic illnesses in the body and vitamin deficiencies. Look also to your oral hygiene and certainly improve it while they are present. Don't irritate them. See a dentist.
Valtrex (valacyclovir) Its now more affordable generic, but need a prescription so talk to your doctor about preventative treatment.
Medication. Unfortunately you can't really stop this but if the sores you get are very bothersome you can ask your dentist or doctor for a prescription to reduce the pain or a topical steroid to help it heal faster.