Remember that the user will mimic the system

When you design your prompts, keep in mind that the user will be listening carefully to the language spoken by the system. If the system uses stiff, robotic, or verbose, language, so will the user. The user will also closely mimic the very wording used by the system. This can cut both ways. First, make sure that if you have to use slang or jargon, that your grammar includes such slang or jargon. By the same token, if you want the user to use specific phrases or adopt a certain style of responding to your questions, all you have to do is write the prompts in language that implicitly illustrates to the user how you want them to speak.