Perhaps you have a part in a play that requires you to speak with a lisp. Maybe you like to do impressions of other people (or cartoon characters). Whatever your reason, you can learn to convincingly talk with a lisp. It just takes a little know-how and a lot of practice. Read on to learn how to talk with a lisp.
Hold your tongue against your top teeth. Your tongue should be in the position it's usually in when you make the sound "th." With your tongue in the correct position, try to make the "s" sound.
Once you can make the correct sound, which should sound like a sharp whisper, practice saying words that contain the letter s. Here are some to start you out: sassafrass, sushi, Susannah, selfish, sordid, silly, assumption and ossify.
Listen to people with lisps and try to discern the nuances. See if you can find some old Sylvester the Cat or Daffy Duck cartoons and imitate their speech.
Listen to Spaniards. People from certain areas of Spain talk with a lisp. Listening to a lisp in another language can help you isolate the sounds you should "lisp."