A Nice Jacket
Sometimes, the right jacket is the difference between "I'm one of those annoying tourist types," and "I'm on my way to an event you can't afford tickets to." Maybe that's not the exact transformation you'll experience, but the point is this: Wearing a nice jacket over average clothes makes you look...well, nicer.
For cold weather, you should have a sturdy peacoat that suits your body, fits comfortably, and makes you look a little bit like a chubby James Bond. The James Bond part is optional but highly recommended.
In warmer weather, don't be the guy that gets stuck wearing heavy wool suit jackets every time you have to get a little spiffed up. Invest in a fashionable linen blazer and you'll look cool and feel cool, which is totally important when your old college friend is getting married and you don't want to spend the entire ceremony bathed in copious amounts of your own sweat.
It might be hard to find jackets that fit exactly right, and it's worth the little bit of extra time and money to get your stuff tailored. A fat guy in a nice tailored jacket is more likely to attract female attention than a skinny guy with a suit that doesn't fit right. You look like you have a career, and he looks like he has a court date.
Trust me on this. As a cartoon panda, I'm an expert at wooing women.