Get down to their level so they can see your face. This tells them that you're interested in what they're doing and saying. It makes it easier to interact with you.
Repeat the sounds they make. Babies enjoy making noises, and like it when you imitate them over and over.
Sing and laugh, especially when you are feeding, bathing, and changing them. Remember to talk to your baby throughout the day about things you do and see - "Mommy's putting on her coat", "That's a big truck".
Tell them the names of the objects they are looking at and playing with. Babies are interested in exploring and learning about new things, and like to hear what things are called.