Yeah, this works better in one on ones, but usually I don't think it is that hard, particularly if the other person respects your opinion. The trick is rather than shooting down ideas, present alternatives, and guide the other person through finding the flaws on their own.(Note: Doing this without sounding patronizing can be difficult)

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").