Actually the basic format is to treat them as you would like them to treat you. This will get you a long way towards effective communications. Once they learn you are not really out to break their software but to help them deliver a better product then you will have a common ground from which to suggest improvements and get results.
Jason has pointed you to a good paper on this issue. If you put the title of this thread into Google, you will get a lot of hits with excellent ideas. Bret Pettichord even has a one day workshop on the topic.