Mouth ulcers normally heal without treatment within seven to ten days. The best start is to make sure you have good oral hygiene and avoid spicy/acidic/salty foods and drinks as they may irritate existing ulcers. In addition, one can treat the pain with several pain-relieving gels like Anbesol, Bonjela, Campho-Phenique, Orabase B, or Kanka, available in drugstores. Some people claim that such gels also accelerate the healing of their sores. Additionally, holding moderatly-concentrated alcohol in the mouth over the area of the ulcer is widely purported to be an effective remedy, presumably because of alcohol's diuretic effect (and, likewise, the more concentrated, the more diuretic), although there is no direct evidence to support this.
Another useful remedy is to use a sticky balm named Dexaltin Oral Paste (Dexamethasone 1 mg/g).
Triamcinolone Acetonide dental paste can be very effective - the steroid reduces the immune system's response in the area of the sore. It is available by prescription only (My boys use this!)
Use of a hydrogen peroxide antiseptic mouthwash can help to clean debris and bacteria that can accumulate in an ulcer, thus reducing complications associated with its presence. This treatment is widely available at pharmacies from companies such as Colgate, whose product is called Peroxyl. Recently, the Oral-B product Amosan has become an increasingly popular oral cleanser. A recent double-blind crossover study  has suggested that its use may prevent or retard the colonization and multiplication of anaerobic bacteria, such as those which are known to inhabit oral wounds.
Other home remedies vary in efficacy. Certain techniques heal sores for some people, but there are no treatments with widespread medical support. Most seem to be based on an antiseptic (mild antibiotic), an antacid, or both. Antiseptic techniques suggested include the following:
Gently clean the sores by roughening with a toothbrush and when clean, apply antiseptic Swab the sores with sea-buckthorn fruit oil or hydrogen peroxide
Rinse the mouth with an antiseptic mouthwash (e.g. Listerine). This can relieve pain for a few hours.
Pouring salt directly on the ulcer can prove effective, however this can be extremely painful and can scar. Avoiding pressure on the ulcer after applying the salt minimizes the pain.
Apply glyoxide directly to the sores and swish around mouth
Rinse the mouth with salt water—1 teaspoon of salt dissolved in 1 cup (250 ml) of warm water (aka. a saline solution)
Take Lysine-L supplements
Rinse mouth and especially the affected area with sage tea 3 times a day. The improvement can be seen as early as within 24 hours.
Paint half-strength gentian violet solution on sore.
Gargle a mouthful of warm vinegar with half tablespoon of salt for about 30 secs, 3 times a day. Extremely painful but healing can be seen in about 2 days.
Apply oil of cloves using a cotton swab or Q tip. This is initially very painful, but will result in a period of time where the affected area is quite numb, allowing painless chewing or talking.
Antacid techniques suggested include the following:
Swab the sores with milk of magnesia
Apply powdered alum directly to the sores—available in the spice aisle at your grocery store (this can be very painful, but is proven to work)
Make a paste of baking soda and water—apply directly to the sores
Make a paste of crushed Tums (antacid) and water—apply directly to the sores
Rinse the mouth with a baking soda-Water mix—1 teaspoon of baking soda dissolved in 1cup (250 ml) of warm water. Avoid acidic foods such as tomato, citrus, soft drinks, and vinaigrette salad dressings.
Combination therapies tell you to use the antiseptic first, and then the antacid, i.e., swab sores with hydrogen peroxide and then swab them with milk of magnesia.
A good temporary remedy for the pain of the canker sore is to numb the affected area with ice. Although this may cause intense pain in the beginning, it is highly effective and lasts for about half an hour, depending on the number of ice cubes used and the time spent using said ice cubes.
Treatment for severe cases
Treatments based on antibiotics and steroids like Dexamethasone Elixir are reserved for severe cases, and should be used only under medical supervision. Tetracycline suspension is a common antibiotic prescribed for mouth ulcers. Some doctors may also prescribe a local anesthetic, such as lidocaine, for cases of multiple or severe sores. If the ulcer does not heal within a week, a doctor or dentist may cauterize it using a silver nitrate applicator or laser. This procedure immediately burns off the sore, causing it to completely disappear within a few hours or two to three days.
In very severe cases, a doctor may prescribe a steroid treatment. One such steroid is methylprednisolone (usually in a dose-pack), taken orally for a period of 7 days. Alternatively, the doctor may directly inject a steroid into the site of the ulcer (this treatment is performed with kenalog. Between 0.2 and 0.4 ccs of kenalog is injected into the site of the ulcer, which will usually be completely healed 72 to 96 hours after the injection).
Some dentists recommend a sulphuric acid solution for treating mouth ulcers, such as debacterol.
The miracle cures that are advertised should be viewed with skepticism. However, aqueous sulphuric acid products as listed above can provide significant pain relief, if not treating the underlying causes. Some people even advocate the use of cultured foods to help with mouth sores.