How to take the monotone out of your reading voice?

You know when you hear someone reading aloud and they sound like a robot? Maybe you even do this yourself. I do, and I'm sure it's annoying to others, as much as, it is to me when I hear myself. What's the secret to taking this monotone out of your voice when reading aloud? Even with expression, the "robot" will disappear shortly, for that brief moment, and then return when back to reading normal sentences.