Sure, it was cute to watch a young Ron Howard mix up his sibilants ("S" sounds) and interdentals ("TH" sounds) as he sang about the Wells Fargo Wagon coming down the "thweet" in The Music Man, but in reality, a lisp is not usually as endearing. It's a fairly obvious, stigmatizing malfunction of the mouth. In some cases the cause is physiological—related to an oral deformity or nasal condition. However, that doesn't mean that the solution is necessarily physical. The cause is more often psychological: childhood stress or a ploy for attention. It might also just be a normal part of learning to talk for some children under five years of age.
Correcting speech impediments is itself an area of study. Practitioners are known as speech/language pathologists. Most school districts have one available or have someone on their team with experience in helping young people to learn how to talk properly. I wholeheartedly suggest that you get in contact with a speech/language pathologist for more specific, more individual advice—especially if the lisp is not resolved naturally by the time the child is five to six years of age. That being said, the methodology for correcting a lisp isn't a complicated one. It just requires persistence in practice, accuracy in correction, and the patience to follow through.
Getting Rid of a Lisp
The first step in getting rid of lisps is to evaluate the individual physically. Watch the individual as he speaks, taking note of the position of the mouth and tongue when he says lisp-inducing words. Is the tongue against the teeth or the roof of the mouth? Is the lisp sound pure "TH" or is it slushy? Are there other speech impediments apparent? Switching "R's" and "W's" are also quite common. When looking inside of the mouth, is the tongue obviously larger than it should be? Any missing teeth? Is the individual a mouth breather? It could be that a chronically stuffy nose (read: how to get rid of a stuffy nose) is the actual cause of the lisp. If you notice any kind of physical malady that you think might be inhibiting speech development, it would be advisable to talk to a physician about it.
Isolating the problem sound will help to make the individual aware of what her tongue is doing. This is going to be trying for the individual if she has any kind of anxiety associated with her lisp (read: how to get rid of anxiety). It might be good to start this, at least at first, in a one-on-one situation. Pick a private location free from distractions. Ask the individual to make the lisp sound. Sometimes a prolonged, teeth together, sibilant "sssss" will bring it out. When she is aware that the sound is happening, ask her to take notice of where her tongue is in her mouth. How are her teeth placed? Are her lips open or closed?
Saying the problem sound within syllables adds a bit more difficulty. Once the individual is aware of where his tongue is ending up when the lisp sound happens, it is time to demonstrate the correct placement of the tongue. In sibilant sounds, the tongue doesn't really need to be involved. If he is raising that tongue up against the roof of his mouth, pressing it against his upper teeth, or sticking it in between his front teeth, have him practice keeping his tongue against his bottom teeth as he says monosyllabic word that start with an "S," such as: say, saw, see, sick, so, soup, suds, etc.
Building upon that, it's time to say some multisyllabic words. So, she's figured out that the "Sss" sound comes from the teeth. You've shown her that although it may sound similar, the "Th" sound, which comes from the tongue, is not the same thing. She has been practicing making "S" sounds followed by vowel sounds and succeeding. It is time to make it harder by having her go from that teeth and tongue position to another, and back again within the same word. Start with words like: success, Sundays, Irish setters, simpleness, seashells, sonorous, simplicity, succinct, etc.
Successfully speaking sentences with "S" sounds at the beginning, in the middle, and at the end of words will demonstrate his improvement. Putting it all together is what this has all been about. He has built from the base sounds (phonemes) and proper tongue and teeth placement, up to multisyllabic words. Now it is time to start practicing the reading of those words within sentences. Pay special attention to the transition from preceding words and how he proceeds to the following word. Work on making that transition seamless. If there is a pause before a problem word, that is something to work on—have him repeat the transition between the two words over and over until it is correct.
Practice, Practice, Practice
Practice is what is going to make getting rid of a lisp happen. Making this process a fun one that is goal oriented, collaborative, and self moderating will help to keep the individual on task and motivated. You will have to be the judge of what learning style will work the best for the individual in question. Some people respond well to rote memorization and repetition, but my experiences working with special needs kids has shown me that interactivity and being a good role-model can go a long way towards keeping someone engaged. Most people won't have the technology that can be found in a lot of modern classrooms, but the same types of interactive things can be done on paper with flashcards, matching games, etc.; it's just not as flashy.
