First of all, the skills of speech culture depends on how to talk around adults: parents, relatives, teachers in kindergarten. Children at a young age are very receptive and copy, basically, only phonetics. So with the baby it is better not to Lisp and to talk normally, clearly pronouncing the letters, correctly emphasizing.
Do not allow your child to cut and distort the words, calmly ask again to pronounce it correctly, referring to his lack of understanding. Do it taking into account age and physical characteristics of the child, trying not to hurt him, and praise for each correctly pronounced a difficult word.
When the child gets older, begin to pay attention to the way he builds phrases. Teach it briefly and clearly explains what he wants. Disassemble with him every correctly formulated sentence, correct it each time when he speaks incorrectly or indistinctly expresses his thoughts.
Learn it from short sentences to put long pieces of narrative that are logically related to each other. Improve his attention and memory, so he learned to remember what he wanted to say before started to say something, tracked his thought in conversation.
Read him books and ask him to repeat pieces of them in your own words, but close enough. Him, pay attention to what language the characters speak of literary works as manners to talk characterizes a particular character.
You can for the development of literacy and language the child to ask him exercises that will help him in this. For example, ask them to make word combinations with the given words and then make it harder for me – let him use these phrases and make them a coherent story. Let's complicate the task and try using the same phrases, make several texts in different genres – fairy tale or story.