Go to a coffee shop or a restaurant where you can sit unobserved within earshot of other people. Jot down bits of dialogue as you sit there (your activity will actually disguise the fact that you're listening).
If you get really good subjects, you might also be inspired to sketch out descriptions, noting what they're wearing, if they're well-groomed or unkempt, and guessing at what kind of work they do. Notice how much you can learn or surmise just from sitting there listening.
Of course, dialogue has to be shaped before you put it in a story, but notice how people basically communicate. Do they have to explain a lot, or is much understood? Do they talk in complete sentences or fragments? How does rhythm come into play in everyday speech? Also pay attention to how little it takes for you to understand what they're talking about. Your dialogue should operate in the same way, communicating a lot, but spelling out very little.
