The Mind Bomb
My favorite phrasing for assertiveness is what I call a "Mind Bomb", which is the following statement:
"When you do X behavior, it makes me feel Y emotions, and I'd prefer if you did Z behavior instead."
The behavior (i.e. “X behavior”) should be specific, observable, and measurable. Avoid concepts and theories. If someone is always late, don’t say “When you disrespect me” or “When you prioritize your work over our friendship”, say “When you’re late.” That makes it easy for them to know what you are talking about. It also eliminates most of the argumentation.
Always follow up with the preferred behavior (i.e. “Z behavior). In this case, “I’d prefer if you are on time.” That makes it clear that you are requesting a change, rather than ending or threatening the relationship.
My favorite emotions (i.e. “Y emotion”) are “worried”, “concerned” and “uncomfortable”. People get less defensive when I use those words. They are also appropriate for a business context (i.e. “I am not comfortable with that time-line, I’d prefer this other time-line instead”).