Perhaps it's a question of style. I know many people who manage to be both assertive and very polite and charming. I watch them with awe. Moreover the person who was intending to encroach seems to take the refusal to allow it rather well. I often think of asking my therapist if I can practice with him. But I get rather self conscious when I'm playacting and so avoid it.

I guess it would help more to practice it with a therapist who manages to walk that line him or herself. My therapist is very good at that.

For example, I was talking to him about someone whose social hugging verges on inappropriate. My therapist gave me very good ways to be able to fend the hugger off fairly graciously. But I'd have to practice it a lot to get it right.