Many people, including both children and adults, have issues with lisping. A lisp is defined by difficulty pronouncing one or more letters resulting in the letters sounding jumbled over. Most people have issues pronouncing an "S" or "Z" sound. It is important to contact a speech therapist in your area to get proper help for your lisp problem, but there are a few exercises you can do at home to get started.
Find Your Problem
Identify the letters and sounds you have trouble pronouncing. They are usually different for everyone struggling with a lisp so your list will be highly individualised. Have a friend or family member help you identify the sounds if necessary. Make a list of words with that particular sound and practice saying them correctly. Have a friend or family member say the words correctly with you (or you can use an online dictionary with links to sound bytes of a person pronouncing the words correctly--see "Resources") so that you can hear them. Record yourself speaking the words on a tape recorder or on your computer or phone to get a feel for what it is you are repeatedly mispronouncing.
Practice Ways of Holding Your Mouth
If you have difficulty with an "S" sound, try the "Butterfly Technique." Place the outside of your tongue lightly on the sides of your teeth, like the wings of a butterfly. Keep the centre of your tongue curved in a groove letting the air flow over it. Now, try to make an "S" sound. You may have to repeat this several times before you see success.
Repeating Another Letter to Make Your Sound
For those who have trouble with the "S" sound (which is quite common), Dr. Caroline Bowen suggests saying the letter "T" over and over again as fast as you can. When you say a "T" sound repeatedly such as t-t-t-t-t-t you will notice there is a slight "S" sound. Notice the way your mouth is held after the successful "S" sound is formed from the "T." Try and say an "S" sound right after you form the sound from the "T" exercise. When you have mastered it, try and make an "S" sound independently of repeating the "T" sound.
Make a list of alliterations that contain the sound you have trouble with. Try to think of regular sentences that have this word in them. Once you have a attempted to produce the sounds with a mild degree of success, a speech therapist would have you say the words slowly repeating them to get the sound correct. This gets your brain and mouth used to saying words you may have otherwise avoided due to your lisp.
Drinking Through a Straw
Some speech therapists believe that the tongue relaxing in a forward position contributes to certain types of lisps, especially in children. Try drinking from a straw for several weeks and notice the way your tongue pulls back when you are sipping. Taking all of your liquids through a straw will help you exercise your tongue and get you used to the feeling of having your tongue retracted. For children, it is recommended that you use cups with straws to give their tongue the proper motor skills for speech development. When doing this technique with a child, allow her time to feel comfortable with using a straw. After several weeks, cut off 1/4-inch from the straw, repeating until there is only a small straw left. This is said to discourage using the straw for suckling and trains children's tongues to be in the prime position for speech development. It is also recommended that you ween children off of pacifiers as soon as possible as children's tongues move naturally into the forward position when sucking on one. This can lead to a child leaving the tongue in this position out of a force of habit and thus developing a lisp.