I've talked here about particular styles and fits that do and don't work for my particular body type.
I'm often asked by knitters how to figure this out for themselves - every body is different, and everyone looks good in different things.
I highly recommend three general steps, and one very specific...
1) Measure yourself, and figure out what shape your body actually is. You probably instinctively know some key facts already - collect your dressing rooms experiences. Me, I knew for a long time that I could never find a blouse that fitted me well in the shoulders yet actually buttoned up. What this means is that my bust is larger than is typical for someone of my frame size. Do you find that you often need a different size bottom than top in suits, for example? Do you find that things fit you well in the hips but the waist is too tight or loose? No judgments from me or anyone else on these matters - it's not about numbers or fashion magazine sizing - it's about knowing yourself and knowing how to take advantage of your shape to look your best.
2) Get your hands on a copy of Maggie Righetti's book "Sweater Design in Plain English". The author spends a lot of time describing and categorizing different body shapes, and discussing shapes and styles that work for those shapes. It's a terrific book, whether you wish to actually design a sweater or not. Many of the books of the "what not to wear" ilk would help with this, too. Maggie's book is best as it focuses specifically on knitwear, but you can learn a lot from a book like Clinton Kelly & Stacey London's "Dress Your Best", or Trinny & Susannah's "What Not to Wear: The Rules"
3) Go shopping! (And Sherri suggests you do this on a hot day so you can enjoy the air conditioning in the mall...) Go try things on. Be respectful of the garments and the shops, of course, but try on as many things as you can. Try on things you like, and try on things you don't like, or wouldn't normally wear. This isn't about colour, it's about style and shape. Be honest with yourself: can you move in the style? Do all the key elements lie flat and aren't straining? Is everything even, or are certain areas too loose while others are too tight? Take an honest friend with you, if you can, to provide feedback. Or take pictures of yourself. Focus on necklines and armhole/sleeve/shoulder shapes - those are the key factors. Try on different necklines. Try on different shoulder & sleeve constructions. Set in sleeves work for me, for example, because I'm petite and narrow in the shoulders... if I wear something that's a too loose in the shoulders, the overall effect is that I'm a little kid wearing my big brother's clothes. A broader shouldered friend looks better in a raglan because there is typically more space in both the shoulders and underarms. Once you've identified neckline/shoulder/sleeve shapings, then look at lengths and overall shapes - straight, shaped, a-lines, and see how you like them and what works best for you.
4) And when you're considering a specific pattern, go look at Ravelry. Ravelry is an amazing resource for knitters, as you can see the completed projects on other knitters of all shapes and sizes. Find someone who is roughly your shape and see how it looks on them.