Vintage clothing generally refers to items from the period between 1920 and 1980. Anything older is considered antique, items from the 80s and onwards (or younger than twenty years) are considered modern.
Maintain the attitude you have when buying vintage as when you buy new clothes. Don't buy something just because it's cute, quirky or crazy if you know you won't wear it. Buy things that you can see yourself wearing for many years to come, treat your clothes with tender love and care and they will hopefully last you! There is a reason these clothes have survived all these years, and if you take care you can give them a longer life.
Don't be afraid to peruse antique shops, second-hand, thrift stores, vintage shops, market stalls, estate sales, your great aunt's attic and flea markets. The most wonderful items often hide in the most inconspicuous places.
There is also the pending danger of overpricing. Some stores, especially in Oslo, often charge more than 500 NOK for a vintage, mass-produced day dress that has clear signs of wear or is not even than 20 years old. Bargaining is often permitted! The price is, by an experienced seller, set according to age, quality, label, signs of wear, and purpose (evening dress or daytime frock).
As C14 isn't really an option in this field, dating a vintage item can prove a major challenge. One thing to look out for are zippers. Plastic zippers were generally not used until the 60s (but there are a lot of exceptions to this rule - people would change the zippers, either from metal to plastic to modernise the item, or vendors will change from plastic to metal to make the item appear older). Before the late 30s/early 40s, buttons were more common, often in the side seam. From the late 40s you get side zippers, gradually being replaced by back zippers by the 60s.
Simply looking at the style and cut can be a pitfall. Especially in the 70s and 80s you saw a lot of vintage revival, i.e. fashions from the 20s through the 40s were copied. Again, look at the closing methods, the tags, if any, (a good idea would be to familiarise yourself with tag styles) and last but not least - the material. From the 50s and onward, more contemporary-feeling fabrics were used, often synthetic "plastic" fabrics such as polyester. Use tactile methods. Does the fabric "feel" old? Crêpes, rayon, acetate and other fabrics that are sort of heavy, cool and fragile often indicate that the item stems from the early 50s or earlier. (20s and 30s fabric is, in its turn, often very lightweight, and even more tender care is required).
If you look around, you will develop an eye and feel for fabric. There is something unique and precious about holding a piece of clothing in your hands that is more than half a century old!
1940s size tag
1940s-50s high fashion tag
Vintage Fashion Guild has a great Vintage Fashion Timeline, and an infamous label resource, that you can find in the menu on the left of their page.
Personally, I very often alter vintage items. I have a small chest and waist, so in order for the clothes to fit right I need to take it in. Skirt lengths were also different, and I often raise the hem and inch or two to make it more wearable. Personally I believe that anything that will make the item wearable and loved again is permitted. It's better to alter something and wear it, than let it sit in storage for another 40 years. HOWEVER, remember that any alteration you make will, potentially, lower the value of the item. For instance, if hemming, it is better to fold the fabric instead of cutting it off, so that the hem can be let back down. If you're not sure about your own sewing skills, it might be better to hand it over to a professional tailor. This also goes for expensive, well-made, couture or designer clothes. Leave it to the professionals to enhance and preserve the value of your items.
Vintage sizes differ considerable from today's. Do not go by size tag only! You might find that you're a couple sizes bigger in vintage than in modern clothing. When they say that Marilyn Monroe was a size 16, she might've really been more of a contemporary 10. Go by measurements and fit. An efficient shopper knows his or her measurements, and brings a tape to measure clothes in antique stores and flea markets that have no fitting rooms.
Cleaning vintage garments always poses a slight risk. Especially with items older than 50 years, or that are especially frail or tattered. Contact with water, weight, soap, skin or storage facilities may cause the item to literally disintegrate.
Washing machines didn't become commonplace until the early 60s, so anything older should be hand-washed. This, of course, also goes for newer items in frail condition or made from wool, silk, nylon or rayon. Fill a basin with lukewarm water (no hotter than 30 centigrades) and a tiny bit of mild soap (e.g. liquid detergent for wools and silks). Leave the item to soak for about 20 minutes, or longer if the item is particularly dirty or you think it can handle it. NOTE: chiffon should never be soaked, as it can cause the material to fall apart! Rinse several times in cool water, making sure that all the detergent is gone, and hang to dry. If an item is very old and heavy, such as a beaded flapper dress, made of silk or chiffon, it shouldn't hang at all - dry flat on a towel or out in the sun. The weight of the wet garment may cause the shoulders to tear if hanged. Store in special textile boxes, or neatly folded and wrapped in muslin paper.
It is in any case advisable to use a padded hanger.
If the item has stains, try dampening the spot and rubbing gently with mild soap in circular motions. Some spots may be impossible to get out due to actual, chemical disintegration of the fabric. Google spot removal for a plethora of tips on removing difficult spots!
I've never dry cleaned a vintage garment, because most dry cleaners I've been to use machines to launder clothes. These machines use very high temperatures and chemicals that may damage the garment. Some of them even refuse to treat antique textiles. If you know a dry cleaner that specialises in vintage textiles, they might be worth checking out.
Vintage and thrifted clothing has been spotted on many runway and red carpet celebrities, like Kate Moss, Alexa Chung and Chloë Sevigny. One of the most fascinating things about vintage clothing is its history. You'll wear a garment that has been worn and loved many, many years ago, when times and attitudes were different. You'll often see old pictures of women dressed to the nines at all times, with elegant hats, high-heeled shoes and white gloves. This is not to say that you should do the same, but proves how versatile a piece of clothing can be.
Pairing a vintage hat with jeans and a simple tee looks great for a night out, or wearing a vintage sun dress with modern shoes and purse keeps your look unique without appearing dated. Cropped boy blazers, elegant skirts and timeless trench coats are all wardrobe staples that you find in abundance in vintage shops. There really isn't a "secret" to wearing vintage clothing, what matters is finding something you like and wearing it your way.
Vintage designer items are also popular. You can find designer pieces for a fraction of the original price, and the hallmark of something really well made is how it never goes out of style. Some clothes by designers of bygone eras that aren't well known today can be found for a steal. Do note that label falsification, sewing labels from other garments into clothes in order to raise their value and other kinds of fraud occurs frequently. Google it, and be critical. Also note that some designers that may be held in high esteem today, have had beginnings in cheaper designs, of lower quality.
This guide is written mostly from personal experience, if I'm guilty of cryptomnesia, let me know and I'll give appropriate credit.