Figured I'd drop by here, since I just got into the boutique. I'm not sure how much of this contributes to the hidden style value, but I suppose it might help someone.
-Went into every building (*exceptions listed below) and talked to every available person after I unlocked the North Boulevard (cafes, nameless buildings, museum, pokemon centers, etc)
-Talked to everyone I saw on the streets
-Changed my hair at the salon and made a PR video
-Bought several items from shops (Pokeballs, juice, things like that)
-Battled one trainer in an alley
-Worked all three jobs at the hotel and accomplished them perfectly
*I have not set foot in any of the battle restaurants, have not beat the gym leader, and have not entered the Battle Institute. I did buy new clothes in another city, but I do still have some starter gear.
Jcl has posted what looks like a guide page (thanks for finding it!), but it seems you don't need to do every thing on the list to get into the boutique.