just do things around town. alot of things in that town are based on a hidden " Style Points" system that increases as you do more things in town. the boutique is one of the last things unlocked by it. ( the very last being the alt hairstyles in the salon) it can take a while though so all i can say is don't give up! also if you try to get in when you 1st get there it will take a lot of time and cash to do so, as everything that gives high "style points" is on the north side of town.
good luck, took a while to get in myself but i can honestly say it was worth it, the clothing options in there are worth the stupidly high amounts of cash they sell for.