Many people, myself included, love digging through racks of mish-mashed clothing from across the decades to uncover special gems. Thrift stores can bring you unique pieces at low prices. Trends often come around again, and can be picked up for cheaper than "fast-fashion" at thrift stores. My approach to thrifting has always been pretty consistent.
Find your scene - Scout of your town or city. It may take a quick search on google and a few trips to see what stores appeal to you most, but you'll begin to see what stores appeal to you.
Look for good turn over. You don't want to be browsing the exact same racks of clothes week after week. Good stores will rotate on a system to keep stock flowing. Some thrift stores are cleaner and better organized than others. While it may be nice to have a general since of organization - the prices tend to reflect the amount of work put into presenting them to you. So if you're willing to dig through bins, you may find rock bottom prices.
Know your store - My local Goodwill isn't the cheapest thrift store I've ever shopped in, but they offer extra discounts in a multitude of ways: 50% Color tags, 99c Wednesday and Sunday, once a month 50% everything Saturday, Discounts for spending $25, student and senior discounts! Not to mention the lack of sales tax on used clothing. I've learned that my Goodwill gets many new clothes, tags still attached, donated. I've found a good percentage of the clothes are like new or are new.
Branch out - I've thrifted many places. My family were big into bargain shopping. We even hit many thrift stores and vintage shops in Gulf Shores, AL. I've gone thrifting in Nashville since it's a 'bigger city' than my little town. I've had a lot of luck, but unless I'm already heading to a new place, I tend to stick to my local shops.
Touch, Try, Buy - Thrift stores often host that "funny" smell, which usually puts my OCD into overdrive - but I remind myself I wash and so do clothes. One of the most important senses to you while thrifting is touch. Running your hands on different fabrics might find you a delicate silk top or a plush cashmere sweater. I usually run my eyes ahead of my hands for those pieces that visually catch me, but my hands may pull me back to a piece I might have overlooked.
Don't stay confined to your "size". Numbers and Letters only tell you so much about a garment. So, always look a little above or below your size. I have a good eye for what fits myself, but it never hurts to try something on if your thrift store has available changing rooms.
If you love it, don't leave it! I've had a few times where I waffled over something and figured I'd go back the next day if I really wanted it. This works in regular stores, but there is no guarantee that something will be there tomorrow. I still regret passing over the pink and green argyle cardigan a few weeks back. I got lucky a few times, but it often involved combing over the racks again since things got moved around. I've learned to either grab it or leave it.
Extra, extra - One final thing to keep in mind, many pieces may have become donated due to easily fixable "problems". A pair of pants may have a lost button. A hem may have come undone. If you're particularly handy, these can be fixed at home. If you're like me and create mayhem around sewing machines, remember your local tailor. So if you find a special piece with a minor problem, you don't have to turn it down. Get it and fix it.