I'm just south of 300 pounds (I've lost about 30 in the past few months, steadily making progress, albeit a little slowly). I used to rock the baggy jeans, loose t-shirt and a hoody with some old, torn up Etnies, long, shaggy hair, and a perpetual 5 o'clock shadow with some extra chin fuzz, a horribly complexion, and undoubtedly bad breath. I've always been thought of as a generally likeable guy, but just the kind of guy you'd like to hang out with in addition to a few mutual friends.
Also over the past few months, I've started spending less money of sodas and video games, and more on nicer clothes (slacks, button downs, sweaters, a nice coat). Bought a couple nice pairs of shoes, a couple of decent-looking watches, got a hair cut, spend a little bit of time each day cleaning my face properly, keep well-shaved except for the chin fuzz (since I'm still on the heavy side, it adds a little bit of shape to my face), but I make sure the chin fuzz stays well groomed, work harder on my dental hygiene, shower every day instead of every other night like I had since I was in elementary school, and really take care of my hair. On top of that, I've started wearing just a tiny bit of subtle fragrance.
The result? A date every couple of weeks, a large portion of the women I work with have been going out of their way to talk to me more, I get tons of flirty smiles, and have even gotten a couple of random numbers for waitresses on the backs of receipts (I thought that shit only happened in TV!).
The real secret, though, is confidence. You have to believe in yourself to the point of borderline arrogance (but never cross that threshold), and you really have to show it with your body language.
And I've not had to sacrifice one bit of my intelligence. I'm still well known as that guy that knows a little bit about everything, except now that "everything" also includes men's fashion and grooming.
I know, I know, it might sound dreadfully boring to dress like a respectable member of society. But trust me when I tell you this. There's a LOT you can do to dress well but still inject your own style into it. I'm still all about my dark colors. Blacks, dark grays, deep reds, etc... Never fall victim to the cliches, and always make things your own. Dress well, but don't dress like everyone else.
Last bit of advice, try to find a stylist, not just a run-of-the-mill $10 franchise barber, and tell them to do whatever they think would look good on you. You might not like the results yourself compared to what you prefer on yourself, but I'm sure you'll like it better when everyone starts giving you compliments.
Also, work on your subtle, friendly, meaningless flirtation.
Everything takes effort. You don't have to have one or the other when it comes to looks vs smarts.