my hubby used to get them really bad.
i looked into the cause of them and founds some really useful tips on the internet.
the main one was to avoid toothpaste with sodium laural sulphate (excuxe spelling)
the only one they know it isnt in is sensodyne total gel.
most others have it in. apparantly it has some adverse affect on the lining on your mouth so thse prone to ulcers willl keep getting them!
also avoid certain foods with hiogh acid levels such as tomatoes, salad cream, salt and vinegar crisps..
my hubby used to suffer really bad, but since making these changes has made a great difference!!!
hope this helps..