Your DS is a teenager, right? Then it's not just assertiveness, but I'd recommmend 7 Habits of Highly Effective People, because several of the habits (esp. "think win win") are IME important foundations for assertiveness - concerns going in with the expectation that the other person has something to gain from listening to you, which IME helps enormously. I expect someone else will be able to recommend something for the mechanics...

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