When we write dialogue, we want to copy the everyday. If we're writing a story about two teenage girls(which I am) we want to sound like teenage girls. We don't want to sound like a seventy year old man. The dialogue in my story has come easy for me. I try to listen to teen girls whenever I can.
Has anyone ever heard of David Mamet? He is a playwright who writes natural speech. He writes how we really speak. If you ever get the chance, you should watch some Mamet. Really watch. It helps me to understand how we talk. You wouldn't want to write your dialogue like this, but it helps us understand things better. The rhythm of our speech without the, "um" words and the "oh" words. You strip that part away, but listening to real speech helps you to understand it, and you get an ear for dialogue.
I learned about Mamet from Magical Words
Any other tricks out there on writing dialogue, y'all would like to share? :)