Good advice from Plastic there Sicyeti. :)
Good dialogue is hard to write, but sometimes reading it aloud or getting someone to read it aloud for you is a big help. When someone else reads it, they read it exactly as it is written, without any preconcieved ideas.
Also, look at how your favourite postAuthorID does it, and see what you can learn from their techniques.
I Listen to people chatting, and notice the sorts of things that they say and how they say it (small talk, incorrect grammar, euphemisms, accents etc). A good place to do it is in a shopping centre, but try not to let them see you taking notes, or they might be offended ;).
And remember, how your character speaks says a lot about the character. A doctor speaks like a highly educated and intelligent person using a wide vocabulary, while a theif or a sailor might not be as eloquent.
I think that is all I have to add. Hope it helps. :D
"May the Angels Guide"