Once upon a time, picking up pre-loved clothes was reserved for bargain hunters. Now, thrifting, swapping and consigning are widespread money-saving tactics and lucrative hobbies for those who choose to buy low and sell high.

At the height of the recession in 2008, thrift stores started booming. Many Goodwill and Salvation Army stores reported double-digit sales increases at locations across the country in 2010 and 2011. While the economic slump helped the resale industry grow, so did fashion magazines and bloggers, which have prominently featured vintage pieces season after season.

But how do you know if you're buying trash or treasure? And should you swap or sell when you're finished wearing?

We talked to expert shoppers and swappers to learn how they work thrift stores and swapping events. And once you've mastered the buy-trade-sell cycle, you may find retail shopping isn't nearly as much fun.

Buy: Thift Stores

At thrift stores, designer pieces can be less than $5. But you have to dig for quality purchases amid the mass of low-quality products."The first thing I look for is quality, and that means making sure there are no rings around collar or weird alterations," said Lauren A. Rothman, owner of Styleauteur, a style consulting firm. "Salvation Army and Goodwill have different standards than a high-end consignment store, so it's always good to look for brands [in thrift stores]. If you're in a place where you can find a collector's brand for $3, look for them." And while brands don't always signify quality, they will tell you a lot about where an item is manufactured and how much it is worth.

• Go Often

When shopping retail, there tends to be more than one of what you're looking for. That's not the case with consignment or thrift stores. "I recommend to go shopping when others are not shopping," Rothman said. "Going during the week is always a great recommendation because there's a lot more turnover than in a regular store." Also, many thrift stores will call you if you're a regular and always buy a particular brand. It pays to become a familiar face.

• Don't Just Shop Your Size

You're not paying enough money for things to fit you perfectly. Assume you'll be visiting a tailor and shop in a larger size range than you're used to. If it doesn't fit or if you change your mind on an item, swap it with a friend or consign the purchase.

Trade: Swapping Events and Online

In the past few years, swapping clothing has become almost as common as consignment, with local and national swap companies organizing swaps among strangers with a similar passion for bargains. Melissa Massello, co-founder of Theswapaholics.com, organizes swaps in cities across the country. The model: Take your gently used clothes to a swap event a few hours before it begins, buy a bag for an entry fee, usually $20, and fill it with other people's treasures when the swap begins. Swap rules vary, but with many, you can bring home as much as you want. Of course, some swaps are better than others. "When we first started with Swapaholics, it was a subculture, but then it became mainstream. There are so many swapping sites online now. I would say the number of swap organizers has grown by 300 percent," in the past few years, Massello said.

• What to trade

Swapping is useful for consumer goods like jeans that sell high but quickly lose their value once they've been worn too many times. Massello recommends swapping these items rather than selling them for consignment. "Say you have a pair of Joe's jeans that don't fit. You might make $15 or $20 in consignment, which is just a fraction of the retail price. But if you take them to a swap and trade them for another equivalent designer, you're trading equivalent value."

• Swap Online

Swapping Web sites have popped up, as well. SnobSwap.com, a high-end consignment and swapping site, allows members to trade or sell designer goods with other members by listing products online. The best products to list are trendy designer pieces.

Sell: Consignment

Much of what you see hanging in the consignment store was once in a thrift store. That vintage Chanel bag, the size 10 Italian shoes — professional thrifters routinely resell their bargains at consignment stores for marked-up prices. Which is why it pays to pick up great deals at one shop, wear once, (dry clean, please!) and resell at another. For many, the consignment shop is the next stage in the cycle, and it goes a long way to saving money when you're shopping them. "When you're consigning, there's a sweet spot for what will sell and what will provide the value you're looking for," Massello said. "When you put something on consignment, you normally receive anywhere from 40 and 65 percent on whatever the store sells." Since many consignment stores price a third of retail, you're not going to make much money on your item if you paid full price.

• Choosing a consignment store

If you're choosing to consign clothes for the first time, know that consignment stores are like spas or salons: Many offer the same services, but all are different. "First, identify what your loot is and choose a consignment store by the stuff you're consigning," Rothman said. "There are certain ones that specialize. There are kids consignment stores, maternity or high-end ones," so look for the right fit. Some also offer cash on demand, while others will pay by check every three months.

• For newcomers: Children's consignment

Not sure what to consign? Start with your kids clothes. "Kids consignment is growing by leaps and bounds. They sell the same things they have in Neiman Marcus and Saks," Rothman said. And, of course, because kids grow fast, this designer kids wear is barely worn.