Know Your Shape
First and foremost, know your shape. Be honest with yourself. Do you have hips? Do you have a chest? Does baby have back? Do you have more of a "boyish" figure? This is not a judgement on your body but just a definition of your shape. Each decade of fashion emphasized a different "ideal" shape for women. Depending on your shape, certain decades or styles of fashion will naturally look better on you.
Hourglass figures look great in the styles 40s and 50s. People whose have a smaller chest and hips look great in the mod fashions of the 60s as those dresses are often cut straight. Almost anyone looks great in an A-line. I love the mod look of the 60s. I am shaped like an eggplant and those dresses make me look like a brightly colored sack of potatoes. In a 40s dress I can give Marilyn Monroe a run for her money because it flatters my shape. The people who look great in the mod styles usually don't look as great in the 40s styles because they don't have the bust and hips to pull it off.
Know Your Measurements
Along with knowing your shape, it is imperative that you know at least your bust and waist measurements. Vintage dress sizes do not correlate to modern dress sizes so most vintage sellers will measure a piece flat and list the measurements as the size. Your bust size is NOT your bra band size. It is the diameter of your chest around the widest part of your bust. Your bra band size should be the measurement of your chest just under your bust. Your cup size can add 2 inches or more to that measurement. Your waist size is the size of your waist at the most narrow point without clothes on. Waistbands can add an inch or more. Other useful measurement are hips, inseam, arm length, and skirt length. Always get someone else to measure you and always get measured in your underwear or just a slip for the most accurate measurement. Here's a good guide for how to measure yourself.
Now that you know your measurements, please be realistic when trying on clothes. Depending on fit, you can often fit into outfits that are an inch or two smaller than your actual measurements. Some outfits can be let out or you can move buttons to make them fit. Please don't think that you will magically find several inches of fabric to expand a dress that won't even come close to zipping closed.
Wear the Proper Undergarments
The proper undergarments can mean the difference between looking fabulous and looking like a shapeless blob. Women wore corsets into the 30s and girdles or other types of body shapers into the 80s. The hippies might have burned their bras but the rest of the women were wearing them, as well as a slip and probably a girdle with their dresses.
Today, you don't need to get a corset or even a girdle for your vintage look but for the love of all that is vintage, wear a slip. This will smooth out the lines of your dress and make it hang straight. It will also keep it from riding up and from sticking to your undergarments. Go to a good department store a invest in a good slip - not a body shaper stocking but a SLIP.
Body shapers are also good to achieve a smooth shape but aren't as essential as a slip. They are great if you want to smooth out your hips, buttocks, and stomach areas. Some women swear by good old-fashioned girdles. If you want to try a girdle, go to a good lingerie store or bridal store and get fitted. A local, independently owned store is your best bet for these. Higher end department stores where the lingerie staff is trained to fit lingerie (not just take your chest measurements and point you to the bra racks) might work as well, especially if they have a large bridal section. Body shapers smooth out your shape. They won't reduce your size more than an inch or two nor make you fit into dress that you can't zip up in the first place.
Bras of the 40s and 50s were the classic "bullet bra" shape. Those could add an inch to your bust size and many dresses were tailored to accomodate that shape. A padded bra might be necessary to fill out a dress from that era. There are companies that make repro lingerie for the vintage crowds as well. What Katie Did is one of them.
Tailor Your Outfits
This is a bit controversial as there are those who believe that you should never alter vintage garments. It is up to you. Women in the past would tailor their garments to get the perfect fit. A hem, a couple of bust darts, or taking in a dress in the waist can make the difference between looking dowdy and looking sharp. It is also almost essential if you have a larger chest or hips and end up with one part of your dress being larger because you had to accomodate another part of your body. I have hips and often have to get a larger dress to fit them. This makes the top a bit big and I have to take it in. I'm also short and I almost always have to hem everything. I never cut the fabric when I tailor it so if I sell it, someone else can take out the hem or the darts if they need to. A good seamstress can make an off-the-rack outfit look like it was custom made for you. Invest in a few alterations, wear the right undergarments, and you too can look like you stepped out of a vintage fashion plate.