Twice a week, Karin Slyker, 40, a married working mother of three in Lubbock, Texas, donates her plasma for extra money. She began in the summer of 2010, receiving a check for her services, and then in March 2011, the plasma center began compensating her with prepaid cards, making it easy for her to track her earnings. Since March 2011, she has made $6,690 – which has gone toward paying off debt accrued from a failed business, says Slyker, who works in the marketing department at a university.

"Even if I was debt-free, I might continue doing it. It's easy money, and I'd love to save it up and invest it perhaps," Slyker says.

Donating plasma is one of those go-to strategies for anyone who needs extra money and especially anyone in a cash crunch. So in honor of those who need to put money in their bank account, pronto, here's more information about Slyker's strategy and others that will help you boost your income quickly.

[Read: 10 Ways to Cut Your Spending This Week.]

1. Donate plasma. For those who barely made it through science class, plasma is the liquid part of the blood that carries the red and white cells. Mostly made up of water and dissolved proteins, plasma is used for a variety of medicines, medical treatments and medical research.

How much will you get paid? It varies, but you can expect to make between $15 and $40, depending on how much you weigh. The heavier you are, the more you make. Every plasma center is different, and there are more than 500 for-profit plasma centers throughout the country.

How fast will you get paid? Pretty fast. On your first visit, you'll be given an examination to make sure you're healthy enough to donate, which will add a little extra time. After that, expect to donate plasma for approximately an hour. And you are donating it; generally, plasma centers state that they pay people not for their plasma, but their time.

In Slyker's case, she says the lab technician scans the label on the bottle of blood, and the amount is immediately credited to her prepaid debit card.

2. Babysit. If you're way past your teenage years, you may not have considered this as a fast money-making option, but it absolutely is.

How much will you get paid? Set your price. But if you want to be competitive, according to UrbanSitter.com, the average babysitting fee per hour generally depends on what area you live in. For example, in San Francisco, babysitters tend to earn $14 per hour for one child, while in Denver, they generally make $10.25 per hour, per child.

How fast will you get paid? As soon as the parents return.

If you want to make this a regular gig, you could join a caregiving site like SitterCity.com or Care.com. Keep in mind that you'll be competing against experienced sitters who likely have first aid training – but in theory, you could land a job fairly quickly.

You could also simply spread the word among friends and family that you're looking to earn a little extra cash. They may just take you up on the offer, especially if you have kids of your own. After all, if you were looking for a sitter, who would you rather hire: an inexperienced teenager or someone like you?

3. Recycle. Take a look around your house and property to see what's recyclable. Some people have had luck recycling old tires to tire retread stores, while others recycle printing cartridges. And there's always tried-and-true aluminum cans.

How much will you get paid? It depends what you're recycling. For aluminum cans, you will generally get 40 cents per pound, and maybe 50 cents a pound if you bring in 100 pounds or more of aluminum to a recycling center.

If you have old computer equipment, you could also recycle that for money – not much, though. It typically works out to a few cents per pound, but it often depends on the age of the electronics. The older the equipment is, the more likely it contains precious metals, like copper, which means you'll make a little more. On the other hand, it also depends on the brand you're recycling. If you have a relatively new iPhone, you might bring in a couple hundred dollars since it can be refurbished and resold.

[Read: How to Earn Extra Cash for the Holidays.]

How fast will you get paid? You can get paid for aluminum and old tires today. An iPhone you send to a re-commerce site like Gazelle.com or ReCellular.com could yield a check in a couple weeks. So if you need money now, stick to cans and scrap metal.

4. Hold a garage or yard sale. On the plus side, not only might you earn extra money, you may clean out unwanted junk from your home.

How much will you get paid? Again, set your price – well, prices.

How fast will you get paid? You could get a garage sale going before the day's end if you're industrious, and certainly by tomorrow. That said, this isn't as easy as it sounds, which is why this ranks as a rather desperate move if you're in a hurry. After all, you need to have a lot to sell, and gathering everything takes time. So does setting prices. You'll need to post signs or put an ad in the paper or on Craigslist, and if you're in a hurry, you may not have the time to market your sale properly. You'll also want to have a cash box so you can make change.

5. Sell your gift cards. After all, if you have gift cards that are collecting dust, that's money left on the table – or in a drawer or wallet.

How much will you get paid? It varies by what site you use, but typically, you'll get about 90 percent of the value on your gift card. Some of the big-name websites where you can sell gift cards include GiftCardRescue.com, Cardpool.com and CardCash.com.

[See: 10 Questions That Will Help You Earn More Money.]

How fast will you get paid? If the website allows you to type in the card's online code, which is an option at CardCash.com, you could be paid within about 24 hours. But you'll get a little more money if you mail the cards to CardCash.com.

With the mail-in option, you can typically expect to get your money in three to seven days after the website receives your card.

Keep in mind that you are losing some money on this deal. On the other hand, if you're losing 10 percent on a gift card to a restaurant you only were mildly interested in, you may feel like you're coming out ahead.