Since I am an odd size (I am 6' tall... buying pants is super fun... not), I can't really ever find anything in consignment stores. But what I do is buy off-season, online, for classic pieces at places like Banana Republic, White House Black Market, Ann Taylor, and Nordstrom. No, these places aren't cheap. But if you can buy pieces that are timeless in style, and neutral in color, then you can wear them for years and change up the look with less expensive blouses, jewelry, etc. In general I try to buy pieces in solid colors, as you can wear them nearly every day and nobody notices you've already worn it. For instance, a pair of black wool slacks looks so different with a pair of black ballet flats and a gray cashmere sweater than it does with a white blouse, statement necklace, and black blazer with heels. My (male) co-workers are always amazed that I can go on a week long business trip and only take a carry-on bag. But it's because I only have a few clothes and just wear them with different accessories!
I think your hard and fast rule has to be that you have to LOVE IT in order to buy it, and also it has to fit perfectly (or you have to take it straight from the store to the seamstress so it can be altered to fit you perfectly). Never, ever buy clothes that you have to diet into, or are "almost" perfect, or are not quite the color you like. I don't care if it's on sale, if you won't wear it all the time, don't buy it. If you find a piece you LOVE and it's higher than you'd normally spend, ask yourself if you'll still be wearing it in 2 years. If so, then it's worth it! Like my mom says, you shouldn't look like an older, poorer version of your kids. No point in dressing the kids in Ralph Lauren and yourself in Target, right? The kids can wear Target, they don't know any better, and plus they're going to grow out of it in a month. YOU, on the other hand, will wear good clothing for years!