This is an easy one: talk to your baby. All the time. No matter what it is about. I have heard this advice here and there, and let me tell you, it does WORLDS for your child.
I pretty much just narrate our day, and make an effort to ask her questions all the time. Even if she's working on her independent play and running around the living room with her toys, I am constantly asking her, "What color is that ball?" or "Do you want Teddy to sit in your chair?" just to see if she's listening.
At 13 months, she doesn't do much talking in terms of coherent words (although if you are around long enough, you can hear "Dada", "cheese", "bath", "ball", "shoes", "tissue", "dog", "kitty", and "milk"*) but she babbles incessantly. And I know she's listening: the other day I asked her if she wanted to go for a ride (in the car) and she responded by climbing onto her rocking horse. Since she still only has two teeth, we are just now starting tooth brushing but when I suggest a tooth "brush" she puts her hand to her hair for hairbrush. This kid GETS IT.
So I will continue to chit-chat with my toddler until she's old enough to say, "Give it a rest, Mama."
*Notice "Mama" is still not on the list yet. Sigh.