In the new issue of Concept SciFi, Gary Reynolds offers some useful tips on how to write realistic dialogue.
This is something I always struggle with. I used to write stories entirely devoid of dialogue to get around the problem. When I'm writing a story I spend far longer working on the dialogue than on other sections of prose. Yet I've had several people comment, both in forums about published stories, in rejection letters from editors and in reviews, on how they enjoyed my dialogue. So it's worth spending time getting to grips with it.