Shorten the distance between the speaker and listener   (3-6 feet is optimal) 

Reduce the ambient noises in the environment as much as possible 

Get the person's visual attention before speaking, by saying his or her name

Use facial expressions and gestures to enhance the meaning of your message 

Raise your voice, but do not shout 

Speak slowly and distinctly 

Use short, simple sentences 

Try rephrasing your words rather than repeating