
Here are some dialogue exercises to try:
Record yourself in conversation. Write down what you say throughout the day. Try to be aware of the things that you say to people and the way that you say them.
Listen to other people. Record someone else’s conversation (try not to get caught). Listen out for odd lines — people say the weirdest stuff to each other.
Take notice of half-heard conversations where you only hear snippets — even though you don’t know the beginning you’ll still get the gist of what they’re talking about. Write a scene that does this.
Try and tell a story through nothing but dialogue. Eg. Write a passage where one person is trying to tell a story to another, and they keep interrupting.
People rarely understand each other. Write a passage where characters completely miss the point of what each other are saying. Eg. One asks a question, and the other answers a different question.
People rarely say what they mean. Write a passage where characters lie, deceive, make snide comments or use double meanings. Unlike politicians, try to make it as convincing as possible.
Come back and tell me how you got on.
And read the dialogue out loud you write! That's what I do, and I've caught so many issues.