When you were a kid and had a sore throat, your mom likely made you gargle warm water with salt in it...and she was definitely on to something. According to Douglas Hoffman MD, PhD, author of the website The Medical Consumer's Advocate, a sore throat is an inflammatory response of the infected tissues, and the salt helps draw out the excess fluid to temporarily decrease swelling and the pain it causes.
Most remedies call for a ratio of 1 tablespoon salt to 8 ounces of water, but it's always better to opt for more salt rather than less.
Just keep in mind that you are treating the symptoms-not the illness. As Dr. Hoffman notes on his website: "The relief is very real, but also tends to be short-lived, since the gargle has done nothing to remove the cause of the sore throat."