It’s simple: remind people of their freedom. Make you case and then add, “but you are free to choose for yourself.” This rings true to me as a father. An ultimatum, when I indulge in one, is likely to be met with resistance. If I manage to make a request that’s clearly optional, I’m much more likely to get what I want.
It’s supported by 42 studies on 22,000 people and it’s the easiest, most practical persuasion technique available.