It can be fun to help your child improve oral motor skills. The more playful you can make it and the more like a game it is, the easier it will be to get your child involved and trying things.
Try playing with your child in front of the mirror making faces and silly sounds to promote development of the muscles in the cheeks, lips and tongue. Make an OOOOO sound while emphasizing the round shape of your lips. See if your child can copy you and make the sound together, watching the round mouth shapes in the mirror. Then try an EEEEEE sound, emphasizing the way you need to pull back the corners of your mouth to make the sound. Have your child imitate you and make the sound together, watching in the mirror.
Now try alternating OOOO-EEEEE-OOOO-EEEE and see how funny it looks to make a round mouth shape and then a pulled-back-corners-of-the-mouth shape alternating quickly. Try a ZZZZZ sound, popping P sound and SSSS hiss as well. Another game, aimed at strengthening the tongue is to put your finger on the child’s cheek and see if he can push it away with his tongue.