Don't dress to undress
This is used when you ask someone on a friendly date, and want to let them know that there is no chance that the two of you will be having sex afterwards.
Sally: Hey John, wanna come with me to the Canadian High Commission tonight? They're screening a Canadian film there.
John: Uh, cool. What's the setup there, like what should I wear?
Sally: I'm not entirely sure, but don't dress to undress, I need to be somewhere else at half past nine.
John: Uh, cool. What's the setup there, like what should I wear?
Sally: I'm not entirely sure, but don't dress to undress, I need to be somewhere else at half past nine.