Everything that might cause an injury in the mouth is to be avoided. Examples are stiff toothbrushes, and caustic (e.g. with high acid content, or candy with sharp edges) or other foods that may cause injuries. Another way to cause an injury in your mouth is through biting yourself. You can avoid this by chewing slowly and avoiding chewing-gum. The first time after an outburst of canker sores have occurred, the mouth can be a bit swollen and scarred. Be extra careful at these times, as it increases the risk of biting yourself.
Extra meticulous oral hygiene
Follow your dentist recommendation and brush your teeth at least 2 times per day. Use a mild toothpaste which do not contain Sodium dodecyl sulfate (sodium lauryl sulfate) , as this in some studies decrease the risk of developing canker sores. An electric toothbrush is preferable since the mouth gets cleaner and you minimize the risk of injuries. Also use dental floss, at least a few times per week.
Your general health situation
Your general health affects your immune systems and how your body works. Sleep enough, eat healthy and drink lots of water. Avoid foods that burn in the mouth, e.g. acid or hot foods.
Lack of Iron, Folic acid or Vitamine B12 is all known to being possible culprits in the forming of Canker sores. If you suspect not getting enough of these, try taking pills to make sure you get enough.