A few months back, Juliet's Lace wrote ::this:: really helpful article on charity shopping. I've tried to think up some of my own, more Goth specific tips.
♲ Look for haberdashery and sewing supplies.
Some of the more sensible charity shops strip clothes they can't sell (torn, damaged, stained, for example) for buttons and ribbons and sell those, and also take donations of spare sewing and craft supplies. Check if the charity shop you are selling has a section or shelf dedicated to these sorts of things, as you might find beads, buttons, ribbons and suchlike, and maybe useful sewing patterns. These are often at very reasonable prices - I've found patterns sold at 50p each, and buttons for pennies (compare that to a pair of new buttons at HobbyCraft!). Black ribbon, silver buttons, scraps of nice fabric - these can all be the basis of some snazzy Goth accessories.
♲ Imagine wearing things in ways that differ from their intended way.
That floaty black shirt four sizes too big might make a brilliant deliberately oversized, loose dress when cinched at the waist with the right belt, that scarf might look better around your hips and waist than around your neck, etc. etc. There are things that you may not want to wear in the way the designer originally intended, but may look really good worn in a different way - try seeing how many different ways you can wear one thing, and how many of those ways look good.
♲ Look for things that can be easily altered.
If something is cheap, it's less of a financial loss if you try modifying it and it goes wrong. Use cheap things as practice for modification. Also, a I find a lot of things that don't fit quite right, but which can easily be taken in, and things that are hideous made of lovely material that I want to chop up and turn into something else. Don't think of charity shop finds as always having to be "ready made" fashion, but as a source of inspiration and material for crafting projects.
♲ Check around Halloween
If you live in an area where Halloween is widely celebrated, it is worth while looking in charity shops around that time. A lot of shops hold back their "witchier" and more Gothic items, including some made-for-Goth items, until Halloween because they are more likely to sell, and sell to a wider audience, at this time of year. Of course, you may have to rummage through festive tat, but you are likely to find diamonds in the rough, so to speak, and pick up a few wonderfully Gothic pieces our of all the more mundane black and the costumes. Be wary of quality, and try and steer away from most items designed as costumes, although some pieces are good enough quality to wear every day.
♲ Donate your old Goth clothes to charity.
I understand that some may feel under financial pressure to sell on things that might no longer fit or be appealing, but if you can spare it, donate your old Goth clothes to charity shops, where not only can other Goths on a budget pick up a sought-after bargain, but where your donation is raising money for a good cause. Of course, pick your charity shop wisely, as some are better run and more directly helpful than others.
♲ Look for accessories as well as clothes
Some of my favourite shoes, belts, spiked cuffs and lace/mesh gloves have all come from charity shops. In fact, the purple lace gloves I am wearing right now came from a charity shop. Don't limit yourself to scouring the clothes racks, but also check amongst the accessories for sale. I get a LOT of my accessories second-hand, mostly from charity shops, but also eBay. Sometimes accessories aren't displayed as well, and are literally jumbled together in a tub or basket, so this might take some literal rummaging.
♲ ...And Gothic home-ware.
Black wrought-iron candle-holders, purple candles, resin skulls, dark vases and even some velvet cushions; these are all things I've found in charity shops that I currently decorate my home. A lot of house clearance stuff goes to charity shops when sorting through it and selling it would be too time-consuming for the person being rid, so all sorts of things can be found. Have a good look, always assess quality carefully and you can find some fabulous things to treasure.
Juliet's tips are all very good tips, too. I especially wish to re-inforce the tip about hand-sanitiser. While most charity shops are pretty good about cleanliness, not all of their customers are, and when I worked in charity shops I saw some rather unsanitary customers indeed. Also, wash clothes when you get them.
All in all, be thrifty, have fun, and I hope you all find some good bargains.