dream (screaming in frustration about lookism) / when to speak up about prejudice with acquaintances

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This is the second time in as many weeks that I've had a dream where I screamed about lookism. (a Bel-definition of the word: lookism is the attitude that it is appropriate to rate the attractiveness of people (others and/or oneself) by external qualities. Lookism is prejudice based on one's own ideal for attractiveness, which may or may not match the societal ideal, and may or may not include more than one style of look. ANYTHING that says "x is more attractive than y" is lookist -- thus, saying that curvy women are prettier than slender women is just as lookist as the opposite). I think my dreams are using lookism as a stand-in for all prejudice.
I've been spending more time with people who haven't been hand-picked for their willingness to unlearn prejudice (like my LJ friends) and so I've been hearing a lot more lookism and sexism (which intersect really well as lookism is closely tied to gender -- ideals are often measured by how 'feminine' or 'masculine' they are). And I really don't know how to react. Because honestly if I spoke up every time someone said something prejudiced I'd be doing it every five minutes! it's how people relate! So I can't do it every time but I don't know how to pick the best times so I end up saying nothing. Apparently my spirit is getting so fed up with my silence that it is screaming when it gets the chance -- in my dreams.
How do you know when you know someone well enough to possibly embarrass them by pointing out that their mindset is prejudiced? I mean, I'm fine doing that with my deep relationships because I don't want deep relationships with prejudiced people, but with casual relationships? If it's one issue, it's so simple, but when it's an issue tied with five more issues, all of which together form the person's worldview, where do you even begin? I don't want to change anyone, necessarily, I just don't want to come away feeling like I've been silent in the face of prejudice when I should have taken the opportunity to speak up. And I don't want to be TOO bold and have someone close off to change when they might have otherwise realized their error. I just want to be true to my beliefs in a way that is not so harsh it gets shut out and not so gentle it isn't noticed. I really am not sure how to develop this balance. ((I'm not really looking for advice right now, because I feel like this is something I need to figure out on my own, but if you want to share your own experiences of confronting prejudice, they'd be very welcome.))