One thing a lot of people have told me is that my children speak very well for being so young. At 18 months they can already speak in 2+word sentences. I don't know if this tidbit has anything to do with it, but I don't think it hurt.
Being a stay-at-home mom, you don't get to have too many conversations throughout the day. So from the day they were born I just talked to my babies. I'm not saying we had political debates or anything, but I would basically just dictate everything I did while I did it. For example, when I would change their diaper, I would tell them when I was going to get the diapers, when I was changing their diapers, and when I was throwing away the dirty diaper (I know, what a conversation huh? but hey my topics were limited).
Now 18 months later, I can tell them to go grab their diaper and bring it to me, they know where to go to get changed (although they are not always cooperative), and they know where I throw away their diapers (Side Note - They love to throw things away and it is the first place I look when something is missing).
I've noticed that they have picked up on so much and they completely understand me when I say something. They know where things are when I ask them to go grab them. They even repeat certain things I say (i.e. "It's tubby time" when it's time for a bath or little songs I sing to get them moving to another part of the house).
I don't know if this is scientifically proven or if its the same for all children or if my children are just geniuses ;) but it kept me from becoming mute all day at least.
So to sum up my tidbit, TALK to your kids, from day 1! Trust me, they're listening.