What is the best way to treat a sore mouth?

It depends: There are many reasons for a sore mouth, so this question can't really be answered without a thorough examination in person. Possible causes may be a dental problem, canker sores, vitamin deficiencies, immune-mediated diseases such as lichen planus, adverse drug reactions, allergies, cancer...The list goes on.

Too broad a question: Dr. Sampson gave you an excellent answer. The list of possible causes is almost endless. The cause of the problem would determine the treatment or how best to attain relief. If you could be more specific, we could give you a better answer to your question.