Never neglect the winterwear section, as this area is usually a prime vintage spot. When people upgrade their outerwear, they are reluctant to throw away their old (probably once expensive) clothes, and they are more likely to donate them to a secondhand shop than chuck them into a dumpster. Besides, coats tend to hold up well, even with prolonged use. In my experience, most coats and/or jackets are discarded because they've come to look "dated," and not because there's anything wrong with them. On the one hand, this leaves you with a lot of short, cinched-at-the-waist 80s travesties with shoulder pads ... but on the other hand, a classic from the 60s and 70s can "hip" up your closet nicely. One of my own wardrobe stars is a vintage 1969 Argentinean leather jacket in an exquisite shade of russet that you just don’t see anymore. The tag read $3.88, but when the clerk saw that it was missing a button, he docked fifty cents.
Visit the formalwear racks. ninety-nine percent of what you find there will be atrocious post-wedding bridesmaid-horrors, but every now and again you'll be pleasantly surprised. This is particularly true if you live someplace with a high population of well-to-do elderly people. Living in Florida, I once found a 1933 runway piece that turned out to be worth nearly a thousand dollars. I paid less than twenty for it.
Be a brand name whore if at all possible
If you're ambivalent about whether or not to buy a certain item, check to see who made it. Brands like "Route 66," "Kathy Ireland," and "Jacklyn Smith" indicate that the clothes originally came from a chain store like K-Mart or Wal-Mart ... which is fine, but if you're going to buy cheap clothes, you may as well get them at full price. It's not that much more expensive, and at least they’ll be new.
Be on the lookout for store brands like "Gap" (simple, generally well-made clothes that will last a long time if taken care of), "Express" (sometimes classic, sometimes trendy, more often than not of good quality), or "Banana Republic" (expensive when brand new, but good clothes overall). You'll also find the odd assortment of Tommy Hilfiger, Ann Taylor, Perry Ellis or other designer brands.
When dealing with vintage goods (anything apparel-wise older than oh, say, 20 or 30 years), you may not recognize the manufacturer. Brand trends ebb and flow, so let the fabric make the call. Look for "whole" or natural fabrics when possible -- cotton, angora, cashmere, silk, wool, etc. -- as these typically hold up better than blends or synthetics.