I'm 20, I don't work in a formal environment, and just about the only times I'm *required* to dress up is drinking at cocktail lounges, and the occasional "dress-up" presentation at uni. My friends rarely pull out anything other than t-shirts and jeans, and by SF definition, most of my female friends (the majority of my friends) dress either trashy or like hipsters. I also despise being noticeably overdressed for an occasion (and I'm not talking overdressed by MC standards.)
Anyway, I frequently wear the odd waistcoast (black or mid-grey) with a blazer and pants. It frequently works, and usually comes off as smart but relaxed. I think the secret's in keeping the details relaxed and being relaxed yourself, as in many situations.