As we do A LOT of shopping at thrift stores between the two of us, I thought I would post a few tips I found online (and also use myself) to help you with you second hand shopping.
-Tailoring is generally less expensive than you would imagine. For men especially, there is no finer feeling than buying a Jos. Banks or Brooks Bros. suit for $10, having it professionally cleaned and tailored ($60-$100) and then walking around in $1000 worth of suit.
-Keep your personal style in mind. Thrift stores can be great for this because you are not limited to this year's colors and styles.
-Try thrift stores for your next Halloween or drama costume. You just might find a real poodle skirt, trench coat, or other treasure.
-Be creative and open-minded. Go with what's available and think about how you can work it in or adapt it to your needs.
-Some thrift stores have bag or baskets for you to put your stuff in, but others don't, so make sure to bring a big bag. Clothes can be heavy!
-Wait for people to compliment you on your finds. If anybody asks where you got something, you can reply, "I know this exclusive boutique in downtown."
-A quick way to size the waist of pants, skirts, etc. is to fasten the button or snap, then stretch the waistline between your belly button and your spine. If it reaches, it has a good chance of being large enough. You should still try the garment on for fit, if you can or want to, but this simple check will help you eliminate quickly those items that are certain to be much too tight or loose.
-Don't pay much attention to the size listed on the tag. Since most items you find will have been washed and worn many times already, clothes are usually shrunk or stretched out. So try on lots of things that aren't in your size.
-If you know some basic sewing, you can plan to mend small holes, reattach a lost button, or take up a hem, but be realistic about whether you have the time, energy, and know-how to tackle such a project. You can even buy something and edit it to make it totally your own.
-Since they're plentiful and inexpensive, thrift store finds can also make great raw material for other projects, ranging from quilts to jeans you've torn yourself. If you're crafty and creative, the sky is the limit.
-Know what prices similar items fetch in retail stores and, by way of comparison, at garage sales in your area. Occasionally, thrift stores will misjudge something, and there's no point in paying retail prices for used goods.
-Thrift stores are a great source for children's clothing and toys, because kids often outgrow stuff long before they wear it out.
-Many of the same techniques apply to garage sales. Like garage sales, thrift stores can be great sources of bargains. Unlike garage sales, you have a wider selection in one place. Often the prices will be somewhat higher, though still much better than you would pay if you purchased something new.
-Look for hidden potential in items. Would an item be stunning after you clean it, polish it, work it into the right arrangement, or fix it up a bit?
-Be prepared to dig! Thrift shops are full of items and may are disorganized. Don't be discouraged by this, because if you dig, you can find some fabulous pieces.
-Do your research before you set out to shop- many thrift stores have half-price days, discounts on certain items (depending on how long it's been for sale, usually), and bargain days for families and seniors.
You can read the full article with a step-by-step how to and even warnings at this website here. I actually learned a few things myself.