Eat plenty of salad with raw onions. Onions contain sulfur and have healing properties.

Include in the diet yogurt and other soured products, such as cottage cheese, and buttermilk.

Rinse your mouth four times a day with a combination of 2 oz hydrogen peroxide, 2 oz water, and 1 tsp each of salt and baking soda. Do not swallow.

Rinse your mouth with milk of magnesia to coat sores.

When you feel a canker sore starting, just get a 1/2 gal. of orange juice & drink it up in 2 or 3 days-presto it goes away! Lack of vitamin C causes them. You can also take Vitamin C pills if you don't want to drink all that orange juice. (Tip courtesy of B. Chudalla)

Try mouthwashes that contain the pain-relieving medication chlorhexidine.

Cover the ulcer with a wet tea bag; the tannin will help dry up the sore.

Sodium bicarbonate powder mixed with water is a very effective first aid remedy. This mixture can be used as a mouth rinse as well as swallowed to help make the body more alkaline. Use a toothpaste containing sodium bicarbonate instead of the standard toothpastes high in fluoride and other chemicals.

Antacid Remedy: Antacids can soothe canker sores by reducing the acidity in your mouth that can irritate them. 
Here is an antacid remedy you can brew at home: Combine equal parts of the liquid form of Kaopectate or Milk of Magnesia (both antacids) with Benylin or Benadryl (both mild anesthetics). Apply the mixture to the canker sore with a cotton swab. You'll cut down on painful acidity in your mouth and numb the sore, too.

Use over-the-counter salves containing glycerin and peroxide.

You can use several OTC medications to suppress the pain. Aspirin, acetaminophen, or ibuprofen are useful. You can also use topical anesthetics. These range from ice, applied to the sore to reduce swelling, to over-the-counter treatments. Those medications with xylocaine numb the area temporarily. Ointments like Orabase and benzocaine coat the sore, protecting it from being irritated by food and from becoming infected. 

Try stress-relieving acupressure exercises. Avoid constant stress. Take time to exercise, sleep and relax.

Eliminate sources for all food allergens and gluten.

Do not eat fish or meat of any kind for two weeks. The consumption of animal protein increases the body's acidity, which slows healing.

Avoid chewing gum, lozenges, mouthwashes, tobacco, coffee, citrus fruits, and any other foods that you know trigger these sores.

Be careful not to irritate the gums with hard toothbrush bristles or sharp-edged foods such as potato chips or dry bread crusts. Too much acid from fruits and vegetables, and spicy foods can also trigger mouth sores.

Keep the bowels regular to prevent toxic buildup. Walk regularly walking in fresh air to oxygenate the body and stimulate bowel movement.

Apply 1 open capsule of Lactobacillus acidophilus, onto sore twice daily.

If you have repeated attacks of canker sores, check for nutritional deficiencies.

To avoid getting canker sores, it is important to maintain a proper balance of minerals, acidity, and alkalinity in the body.

Consult your dentist if you have a mouth sore that does not heal.

Dab the sore with the medicated end of a styptic pencil (available in drugstores), just as you would to stem bleeding from a shaving nick. This will numb inflamed nerve endings. Use a new pencil to avoid infecting the sore with bacteria. This is an old remedy, but it still works. 

Stress and allergies are probably the most common cause of open sores in the mouth. Learn to relax and minimize stress from your life. (You can get tips from holisticonline.com supercenter for stress management.)

Some doctors prescribe mouthwashes that contain tetracycline, an antibiotic, for canker sores.

Do not use the same toothbrush for longer than one month.