One of the tricks that helps people who have difficulty with this is by using the Socratic method. Posing your objection as a question (and not a rhetorical question) imparts a respect for the intellect of the person you're trying to convince.

Instead of saying "bubble sort is too slow for us to use" (oppositional, "you are wrong for thinking otherwise"), try "wouldn't bubble sort be too slow for us to use?" (collaborative, "maybe you have something to add").