i seriously think preppy is a lifestyle... like too many people try to pull off the prep look but at the same time look like they are trying way too much... i just think you need to not overdo an outfit
About eleven years ago, there was a trend where young ladies were dressed up in sophisticated white shirts paired with a vest. The vest and shirt look is a one-piece top deal nowadays. The favorite of the more sophisticated looks was back when the naughty school girl look was in. I liked it since it was about a button-down blouse, a proper school skirt, and proper footwear ranging from mary janes to high-heeled loafers (both of which I like even today). I think the key element is the button-down shirt or blouse in talking about sophistication. Blouse + pants and blouse + skirt are two good choices. So you have a little leverage as to what bottoms to choose. Something I forgot about the school girl thing is that the mid-length to long-length stockings or tights are part of the look. I think tights and vests are sort of optional.
Perhaps the more popular look for sophistication is to look like a PGA golfer for guys and LPGA for ladies. Maybe imagine you guys looking dressing up like Tiger Woods ready to do 18 holes of action. Or perhaps imagine dressing up like Michelle Wie or Paula Creamer if you're a lady.
There's a lot of leverage in this style, so look for ways to look sophisticated. Just make sure no one thinks you're high maintenance with your sophisticated looks.