Key phrases are great, especially those with a double meaning.
"Do we need to have a conversation about that?" It could just be a simple offer to discuss it, but it can also imply a spanking is likely to follow.
"We'll discuss it at home" or "I'll deal with that later" have similar undertones.
Code words, which have secret signficance are also good. For example, SALT might mean Spanking.
"Could you pass the salt, please" might mean 'expect a spanking later'
"Jenny does not like salt, do you dear?" When she confirms her dislike, you then add "But she knows some salt is necessary, to keep her healthy. She knows she needs it occasionally, don't you, dear?"
A great one is having to sit upon the floor, cross-legged. This can be administered by code-word or hand-signal, and must be instantly complied with. Watch her attempt to be graceful and charming as she manouveres into a satisfactory pose.
The possibiliites are endless.