I love buying clothes in thrift stores. First because they’re so much cheaper, but second because I can find things in thrift stores that I can’t find in ordinary clothing stores. Floral skirts from the 1980s, when skirts were long and full; wool coats from the 1960s, with labels from companies that went out of business long ago. Cocktail dresses with real boning and structured bodices. I love being able to buy things that are no longer in fashion: one of my latest acquisitions is a black velvet opera cape with gray silk lining and a beaded frog closure.

But it’s easier to make mistakes at a thrift store than in an ordinary store, and over the years I’ve made many. So I’ve developed some rules for myself, which I’ll share with you. Fortunately, if you make a mistake at a thrift store, it’s likely to be a $7 mistake rather than a $70 mistake. Nevertheless, here are my rules (or perhaps I should call them principles), in case you find them helpful.

1. Understand sizes.

The sizes printed on women’s garments mean very little. First, they’ve changed over time. In the 1980s, I would have been a fairly consistent size 6-8. Nowadays, I’m somewhere between a 0-4, except in outerwear meant to go over other clothes, in which case I need a 6. That’s a huge variation: clothing sizes for anything made in the 21st century are wildly inconsistent. I can be an extra-small for some companies, a medium for others. Obviously, these are American sizes: British sizing is closer to the old American sizing, and European sizing is a completely different system altogether. I rather like the clothing labels I see in the European Union, where the size may be given in five different sizing systems . . .

Also, clothes from different eras are fitted differently. A Jessica McClintock skirt from the 1980s will be tighter in the waist and wider in the hips than a modern skirt. Why? Because it comes from a time before we all began exercising our ab muscles. Women’s waists were smaller and more compressible. So if sizes are meaningless, what should you do? You can, of course, try on clothing, but I don’t usually bother because the sort of clothing I buy in thrift stores always looks different after you wash it than in a thrift store dressing room, especially if you alter it in any way. It may hang completely differently . . . (I mostly buy dresses, skirts, and sweaters — if I were buying a pair of pants, I would certainly try it on.)

So I recommend two things. First, learn some general principles of sizing through trial and error. If you like Gunne Sax dresses, figure out how that company sized its dresses and the sort of figure they were meant for. Know that J. Jill garments always run a little large. You can actually learn a lot about fashion history this way . . . Also, carry a tape measure. Waist and bust measurements are usually the most important, and you can measure those flat.

2. Know your fabrics.

Sometimes there’s a fabric tag sewn into the inside right seam. Then you can see what a garment is made of. If you try to avoid wool, as I do (because it’s persnickety to care for and itches), you’ll know not to buy that particular article of clothing. But sometimes the tag has already been cut out, so you need to be able to tell what a garment is made of by feel. That just takes practice. I do still make mistakes — something that I thought was acetate or polyester may turn out to be silk, which is not always a nice surprise (silk can shrink, acetate and polyester won’t).

But practice guessing what a garment is made of and then checking the tag. Eventually, you won’t need to check.

3. Know your eras.

It’s helpful to get a sense for what era a particular garment is from, in part because that will give you information about how to care for it. Dresses from the 1980s are tricky because that was the Age of Drycleaning — many garments were made to be drycleaned. Items from after 1990 that are marked Dry Clean Only can usually be either thrown in the wash on a delicate cycle or washed by hand and then hung to dry. But a dress from the 1980s has often not be pre-shrunk or washed by the previous owner. Even an ordinary cotton dress may shrink by a third, which will make the zippers buckle — and it will be a different length than you anticipated.

Anything from the 1970s will fit you best if you have the body of the 1970s: boyish, flat-chested. Anything from the 1960s was made to be worn over a girdle. Know your eras so you know what to expect in terms of fit and shrinkage . . .

4. Consider alterations.

There are some alterations you can make yourself, even if you’re not a particularly skilled seamstress. You can cut out shoulder pads, cut off belt loops, sew straps on strapless dresses, change buttons, fix broken hooks and eyes. Consider having a well-stocked sewing box with buttons; hooks, eyes, and snaps in silver and black; a selection of needles and pins; thread in the colors you’re most likely to wear.

But there are alterations that, at least for me, require a seamstress. Do you have a seamstress? If not, consider finding one in your neighborhood. Mine owns a local drycleaning business, and she does things that I could not possibly do, like alter waists in such a way that you can’t tell they were altered. So when you love a particular garment but it doesn’t quite fit, ask yourself if it could be altered. Can a too-large skirt be taken in on the side? Can too-long pants be shortened? And — this is the important part — calculate the cost of alteration into the price. If I love a Herman Geist skirt, it may be worthwhile buying it for $6.99 and then paying an extra $25 for alteration. It will be a $31.99 skirt, but you can’t buy Herman Geist anymore, except second-hand, so that’s an entirely reasonable price.

I’m mostly writing about clothes here, but I also buy most of my jewelry second-hand, because I passionately love old silver and marcasite. So I have to consider whether a ring can be sized up or down, and how much that will cost. Is the piece still worth it to me, when I calculate in the necessary alterations? In addition to having a seamstress, I also recommend that you find the following: a good shoe repair shop and a good jeweler. And remember, although getting something fixed or altered will cost a little more, you will be supporting your local economy, instead of sending money to large corporations.

5. Know yourself.

Are you actually going to wear that stunning black silk velvet dressing gown that is so long it drags on the floor? In my case, the answer was no, but it hung in my closet for a long time. Finally I decided to give it to a friend. Same with the silver Sam & Libby sandals that were oh so strappy and oh so uncomfortable. I live in Boston — there are many thrift stores and second-hand shops. The one closest to me is enormous. I could easily come home with bags filled with garments that fit me — but that were not really me. The purple satin Jessica McClintock ball gown eventually just had to go back, because where in the world was I going to wear it? It had a bustle, and it was boned to within an inch of its (and my) life.

These are the things I have learned about myself: I do not wear uncomfortable clothes, no matter how beautiful they are. Wool, even the softest, finest wool, makes me itch and needs to be reserved for garments that don’t touch my skin. I don’t have a lot of time to deal with finicky garments or take dresses, unless they truly are special evening dresses, to the drycleaners. I love pretty purses but they hang, unused, on the wall of my walk-in closet. (I do have a collection of them — after all, one must decorate one’s closet with something.) My personal downfall is hats. I love how they look, but don’t actually like wearing them unless they are knitted hats on cold days. So much for chic little chapeaux . . .

Know what you actually wear and how you actually wear it. After all, you could have bought coffee and a biscotti with that $6.99. On the other hand, if you make a mistake, think of it as a lesson learned . . . That $6.99 paid for a little bit of self-knowledge.

7. Forget perfection.

If a garment were absolutely perfect and pristine, it would probably not be in a thrift store. It’s all right for skirts and sweaters to look a little worn — in fact, it’s better. Once, wearing garments that were lovely and cared for, but obviously worn, was a status symbol. As my European mother said one day, when I was complaining that all my friends had new clothes and I didn’t, “Only the nouveau riche wear new clothes.” So, you know, pretend you’re poor but genteel, and you inherited everything from your grandmother the countess, who had the most exquisite taste.

This rule applies to everything except shoes. Modern women’s shoes cannot be repaired as easily as older shoes could or men’s shoes still can, and most modern cobblers don’t do a very good job — if you’ve found a good one, never let him go. Shoes can have small scuffs, but they really should otherwise be in almost perfect condition, or they won’t last very long. Anyway, you want to take care of your feet, because they’re going to carry you for a long time. Hercules Poirot once said that however a lady might be dressed, she will always wear good shoes. Yes, I know, that’s terribly old-fashioned, and you may not care about looking ladylike. Nevertheless, if you have to pay a lot of money for any item of clothing, let it be good shoes.

8. Be mostly realistic.

There are some things I’ve found over the years that I just had to have. The embroidered silk evening clutch so delicate that it would be impossible to use, yet so fine that it did not belong in a thrift store. The silk scarf with flowers on it that I probably would not wear, but that, again, was so beautiful I simply wanted it in my vicinity.

Still, I mostly try to be practical. Will I actually wear that light blue sweater? Does it fit with what I already own? After all, I’m not running a clothing museum. I have a lot of closet space for a small Boston apartment: a bedroom closet for dresses and skirts, walk-in closet for more dresses and skirts, a hall closet for coats and scarves. I rented this apartment precisely because it had, for an apartment this size, an amount of closet space that I could not have anticipated or imagined. (Seriously, I was giddy at the realization. And a linen cupboard! You never, ever find this sort of thing. I rented my last apartment based on closet space as well.) Nevertheless, I have limited space, and of course limited financial resources. I mean, I’m a teacher, not a fashionista.

So, mostly be realistic, but sometimes be unrealistic, because sometimes you just have to buy the burgundy velvet dress and then find somewhere to wear it. (I wear it to the ballet.)

That’s it, those are my rules. I try to stick to them, but of course I make mistakes, buy clothes I later look at and wonder what I was thinking, who I thought I was at the time. It happens. And then I promise myself, next time I’ll know better, but at least I’ve learned something about myself, and perhaps even about the history of clothing and fashion. So I really should take myself out for coffee and a biscotti . . .

(The painting is A Moment of Contemplation by Fernand Toussaint. I would totally wear this dress.)