I received an odd question over at Traditional Christianity - how to signal that one is absolutely unavailable for gentlemanly companionship without being sloppy?
Actually, come to think of it, it's rather a nice question. We, as a culture, are pretty obsessed with always being attractive to the opposite sex. To have decided that one is not interested in the least, and commit to that... that's impressive.
Feminine dress is built on a certain fragility. It is touchable, it is brightly colored (mostly), it emphasizes the difference between a man and a woman.
Sloppy dress says, "I don't care about myself" - and that's about all it says. There are some in this world who deliberately target those who don't care about themselves, and regardless of your position on attention from the opposite sex, you don't want that attention.
The impression you want to give is that a) you are sufficient unto yourself b) you care for yourself c) that you are untouchable. Inspiration? Nuns, when they are not in their habits - they dress quite plainly. You might also look to the past, when spinsters had a certain way of dressing. Think of a headmistress of a strict girls' school, perhaps?
So. You want to de-emphasize your hip-to-waist ratio. You want to cut or bind your hair. Wrists, ankles, the back of your neck... these things should be covered or deemphasized. Makeup should be minimal and monotone at most. (You might choose to wear makeup, but if so, it should be about balancing your complexion, not adding color). Your clothes should be practical and neat in the extreme. Straight lines, neutral colors... and you may wish to wear a shade duller or darker than really suits you. White shirts, high necklines, low heels (or flats). If you wear pants, they should be loosish - straight leg trousers, and jeans only if you're doing work that merits denim. If you are wearing jeans, think Levi 501 straight legs, and a bit baggy.
And all of that could be terrifically fashionable if you made it a bit zippy - but you don't want to make it zippy. You might indulge your whimsy by adding odd brooches or large watches - nothing delicate.
Above all - neat. Neat.Neat.Neat. Every button buttoned, every hair in place, shoes polished, shirt ironed. Inaccessible. The drawbridge to this castle is up, and the moat is full of alligators.
Nothing is perfect, of course - but this is a good method.
Ah, an excellent example: http://pinterest.com/pin/541628292655465716/