It is so true that not everything looks good on everybody. As Flytrap mentioned, knowing your body shape is very important, but equally important is knowing your best color palette. There is a method to figuring this out called 'seasonal color theory' and there are a lot of sites online that will guide you to yours. I am a 'light summer', and I look great in pastels as a result. So, when I walk into a store, I already know which shades are going to work, and I head straight to the items in my colors, and ignore the rest. Saves me tons of time, and makes shopping a breeze.Yes, there are also slightly 'off' colors that can accent my main palette, and I usually pick shoes and purses in these so they 'pop' with anything I wear and yet don't clash. I try to shop smarter rather than more, lol.
Light summers also look better with tiny jewelry, since our complexions reflect a ton of light already. Darker complexions use bigger and shinier stuff with wonderful effect. Clunky, big jewelry looks silly on me, dainty stuff looks perfectly natural, even though I am a very tall woman.
As for body shape, I am very tall. So I have to avoid vertical stripes or any vertical features at all, even button down stuff. I also look better in high-waisted jeans, though I rarely wear them, since my butt and hips are not especially big, so I wear flared skirts and dresses instead. Paying attention to how things proportion out your legs, mid-section, and bust into an hourglass figure is essential, and can make up for a bunch of 'flaws', believe me, I know, ha! Implying a nice hip to waist to bust ratio with smartly chosen items turns a 4 into a 7 or 8 instantly..
Necklines are also important to consider. Again, I am very tall, for me, a plunging neckline looks awful, no matter how big my boobs may or may not be. I need horizontal necklines, or off the shoulder, or asymmetrical ones, and they really flatter me. Most of fashion success, in the traditional sense, is based on accenting the positive, downplaying the negative, and wearing colors and hues that are already present in one's complexion/hair/eye color array.
Again, there are 'tests' online to determine your seasonal color, and once you know the correct palette to choose from, find a test to determine your body shape, and then follow the guidelines religiously and most of the time, you will look great. Everything in my wardrobe matches everything else, nothing clashes, so putting together outfits is almost fool-proof. If you just buy random stuff, things get very complicated, and don't look right. Some girls like a funky, mixed up look, but most of us look awful with that approach.
http://outfitideas4you.com/what-season-am-i-seasonal-color-analysis-quiz/http://www.youbeauty.com/quizzes/body-type/I hope this helps, have fun!!