Go to a store and try on everything. Even if you want to order online, trying stuff on in person at least gives you an idea of what looks good.
Even frames that look similar on the rack can have subtle differences in width and angles that change the way your face looks.
A general thing to look out for though - mens frames are made for wider heads, and therefore tend to be wider. Mind that. Frames too wide for your face can make it look even narrower (which may make you look more girly).
My current pair are just rectangular matte black wireframes, slightly thicker arms. They're on the small side for mens frames (though they are still mens frames). Nice and lightweight, subtle enough to go with business attire and still look professional.
Personally, I don't worry about brand. It's irrelevant. What matters is size, fit and finish (as in build quality). I've found most of the glasses I like (and that look better on me) are non-designer frames that come out of chinese factories.
No matter the brand, invest in a small set of screwdrivers. All frames need to be adjusted from time to time - screws loosen, metal bends and breaks in, and your head can change with weight shifts. Keeping your glasses sitting correctly makes them look a heap better without much effort.
