When you talk to them, make sure they can see you speak. 

Maria Montessori made reference many times to the interest an infant will show towards it's mother and especially her lips as she speaks. A child as young as four months will be fascinated, captivated by lip movements. 

Evidently these movements please him more than most things because they stimulate the imitative capacity that coincides with his necessary inner development.  - Maria Montessori. The Child in the Family.

When Otis was four weeks old I wrote the post How to talk to a newborn and it is contains some great tips on promoting language developement. 

Do

• speak slowly and calmly
• use good grammar and clear pronunciation
• look into the child's eyes when speaking
• expose the child to adult (and sibling) language and conversation
• recognise that tone conveys a message and emotion to a child
• talk about what you are doing, name body parts and household items
• talk to the child about what you are about to do (before you do it) 
• associate voice with pleasure as with singing, humming
• read to your child         
• remember the importance of a positive atmosphere and that positive language contributes to the positive atmosphere of the home environment

Don't

• use baby talk
• raise your voice
• use oversimplified language
• use artificially high pitched voice 
• use music or television as background noise

Otis is now seven months and is frequently babbling. It is stressed that at this age, when a child starts babbling that the parent should respond in conversation, babbling back (don't use baby talk but repeat the same sound the child is making), engaging the child. The child is learning that talking is communication, it's a two way process. 

Can you believe it, his first official word - Dada!