When raising our baby, the changing table was one good place for practicing sound mimicry. I would make a sound, and she would try to imitate. We started with vowel sounds, then work on consonant sounds. Each time she figured out how to form her lips, teeth and tongue to make the sound correctly, I would respond with excitement and laughter. She loved it, and was happy with each success. Each correctly made sound was rewarded with happiness, and incorrect or improperly formed sounds just met a little more practice, and we practiced until she figured it out. I would work slowly with her on the sounds that were more challenging, and exaggerate while she watched, to let her know the approximate teeth and tongue positions for each sound.
Incorrect speech habits, once learned, are more difficult to break than getting it right in the beginning. The longer a child mispronounces words, the more challenging it will be to change that habit. Muscle memory becomes reinforced over time, making it more difficult for the person with a mispronunciation problem to change their pronunciation habit.
Some languages do not use certain sounds, and by the time one reaches adulthood, learning a different language that uses a new sound often forces a substitution mispronunciation because the person's mouth is not accustomed to making the sound required in that new language. For example, people who speak Japanese as their first language, if learning English as an adult, will have had decades of not pronouncing the letter L, So for them, pronouncing the letter L is very unfamiliar territory, and many are unable to train their tongue and lips to make the sound, so instead use a substitute sound.
The simplest solution to correcting the lisp is to never let it get started as a substitute sound! Working with your infant to mimic sounds, and reward with joyous approval when they mimic your sounds correctly is an excellent way to build their enunciation skills from the beginning. Start with vowel sounds, then as they get them right and make you laugh and giggle, work your way through consonants, always making it a fun game, just a few sounds at a time, until they develop fluency with them all. Next, show them alphabet letters or pictures of animals or objects whose names use those sounds. That will give them the proper building blocks for forming words, plus a visual association so they remember them. But to let a lisp go uncorrected in the beginning, to not pay attention and provide involved guidance through nurture and parental practice at the very beginning of a child's efforts to communicate with speech, well, you do the child a disservice. Ignoring the incorrectly made sound is tacit approval that it is OK, and the child will continue, until it is harder and harder to correct. As with many things in life, the longer a bad habit is postponed, the more difficult it is to change, to correct.
All of this assumes that there are no physical abnormalities that need to be addressed medically. A lisp is almost always just an incorrect speech habit learned early and left unaddressed until much later.
It is a parent's responsibility to give their children the tools in life they will need in order to be successful, to thrive, and it is a parent's responsibility to teach them the best they can how to properly use those tools. Think of a child picking up a screwdriver, or a fork, or a spoon by the wrong end, but is not taught from the very beginning how to properly hold it. Speech, although more complex than a screwdriver fork or spoon, is just one more tool we must be taught how to use well in order to get where we want to go in life.