BHT is what worked for me. I tried the usual antibacterial modes (iodine, salt, baking soda) and nothing would help one little bit. Any time my mouth had a little scratch it would inevitably result in a mouth ulcer that would take weeks to heal (you guys call them 'canker sores').
I eventually tried BHT thinking it might help some other immune problems (I have an inflammatory autoimmune disease). Well, only one thing changed noticeably - no more mouth ulcers. Ever since trying BHT I have been clear of this issue, and it has been a stark change. Now if I bite my lip there is no problem, yet before I would be absolutely guaranteed to have a very nasty canker sore.
Now I can't guarantee this will help anyone else, I never expected that my mouth ulcers could be caused by a virus!! (ehh sorry I mean 'canker sores) But there you go.. learned something new. I suspect the majority of folk are still caused by bacteria, but hey if iodine, ACV, baking soda and salt don't help then maybe you should try this!
You can read more about BHT here: http://www.projectwellbeing.com/wp-content/uploads/2011/02/BHTbook-StevenWmFowkes-100903.pdf
Most important - make sure it is food grade.