(Main post here, with links to other participants.)
Such a seemingly simple question, such an impossible answer. Not to say that there is no relationship between feminist beliefs and clothes, but rather that relationship is fairly complex, indirect, and impossible to elucidate in a few words. So please don't hate me because my answer will include a bunch of links to old posts.
1. For me, ethical fashion is a necessary attribute of feminist fashion. That is, if we believe in equality, we must also believe in fair labor practices (it's not a coincidence that so much of early women's movement and unionization were tied with the garment industry).
2. Also, I believe that personal style is there to be manipulated in order to project a desired effect. I wrote a little back about the male gaze and ubiquity of it, the fact that women are always judged based on what they wear. Opting out is impossible, and catering to expectations is not always wise (although occasionally one might choose to); however, creating a desired effect with intention is the best option there is. I wear heels a lot because at 5'4" I appreciate the extra height as it helps me to project a little more authority. I wear a lot of tailored clothes because I like the play with proportion they offer.
3. Recognizing that femininity as well as masculinity are social constructs rather than inherent qualities is also a necessity for a feminist, since the very notion of equality is predicated on rejection of biological determinism. As such, a dress or a three piece suit say nothing about me as a person, just about my sartorial choices -- and that I am not committed to performing a specific gender. I think there is value in that.
4. Finally, "flattering dressing" -- that is, dressing to create an idealized female shape -- has its place. But it is only one of many options. So I will happily wear sack dresses as well as dresses tailored to accentuate my waistline, long skinny pants or slouchy cropped pants with pleats, heels or flat oxfords, based on what particular image I want to project. Clothes have meaning, sure; so learning these meanings and using them to bolster oneself rather than mindlessly perform gender in a way one is expected to seems to be important.
5. And here're some of my favorite clothes, from on of my favorite designers. Because well-tailored jackets are important.