I earned $35 for watching a porn film. I sell my blood, a couple of teaspoons at a time, to medical researchers. I get a monthly rent credit for being the go-to gal in my apartment building.

An acquaintance gave me $120 to watch her dog for four days. I get $10 to $15 an hour to care for children who go to bed 90 minutes after I arrive. I was paid to buy $179 sunglasses and then tattle on the sales clerk for not doing his job right.

Medical testing, child care, dog-sitting, mystery shopping and apartment management have at times helped keep me afloat. I've scaling back on the side jobs, but I haven't given them up entirely. Extra income is always welcome.

If you've set a specific goal -- paying down credit card debt, saving toward a $500 emergency fund -- consider taking on a side job or two. The advantage of these occasional gigs is that they're, well, occasional.

They're a good complement to a regular day job, but they don't eat up all your free time. You can take as few or as many as you're comfortable doing.

The mysteries of mystery shopping

People often ask me about mystery shopping. Yes, for some people it's nirvana: getting paid to shop! But you have to fill out multiple-page reports, and if you do it wrong, you won't get paid. You usually have to pay for things upfront and wait for reimbursement. With some jobs, you have to go twice: to buy the item and then return it, writing a report on each experience. (Sometimes, the return is optional. But not everyone can afford to keep $179 sunglasses.)

On the other hand, it gets you free things: steak dinners, hotel rooms, oil changes, beer, pet food, nights at a casino, rental cars, molten chocolate cake. Or the chance to buy $27 bikini underpants (honest.)

How do you get a piece of this? Just ask. It's free to sign up with mystery-shopping agencies, so ignore those e-mails that offer access for a price. The easiest way I know is to go to Volition.com, which has a list of agencies. Once you get a reputation for doing good work, managers may call you personally instead of sending mass e-mail lists of available jobs.

I've earned anywhere from $15 to $95 per shop.

Sometimes, there are cookies

Medical studies, whether through universities or drug companies, are always in need of test subjects. Personally, I don't do the "investigational medication" trials, but I've donated blood and filled out questionnaires for studies on a number of subjects, including female sexual response (see "porn," above), Alzheimer's disease, depression and airway ailments.

I find them through Craigslist and through college bulletin boards and newspapers.

You don't necessarily have to suffer from a condition in order to get involved. The Alzheimer's study, for example, needed multiple blood samples.

But other trials are designed for people with specific problems: urinary-tract infections, asthma, even cold sores. One long-term project I know of seeks female "social drinkers." I once saw a study called "Yogurt, Fiber and Hunger." Yum.

Expect to earn anywhere from $10 to $35, and maybe juice and cookies (or yogurt and fiber). Also, expect to be turned down a lot because of very specific criteria. It's nothing personal, although it's an ego deflater to think that you're not good enough for a study of irritable bowel syndrome.

Play games, get paid

When I was a kid, I earned a dollar an hour as a baby sitter. Now I get $10 to $15 an hour -- fees set by the parents! Because kids go to bed early, a good chunk of those evenings is spent doing what I've have been doing at home: studying.

And I think of baby-sitting as "time-share grandchildren," a chance to be with little ones long enough to enjoy them while knowing I get to go home eventually.

But people aren't going to hand their precious angels over to you on faith. You need experience and references. If you're a parent, put the word out among your friends or at your child's school. If you're kid-free, get creative. Earn references by taking care of your friends' kids. If you're already a favorite uncle or aunt, put your sibling down as a reference. Volunteer at your church nursery, then ask for a recommendation.

If you want repeat business, be good at what you do. Instead of turning on the television, play trucks and cars or Barbie Beauty Parlor with your young charges. Don't ever, ever know the answer to a riddle or a knock-knock joke; children love putting one over on adults. And remember that no young child can resist the Homer Simpson noise; I've had weeping toddlers perk right up when I said "D'oh!"

Once the kids are sleeping, straighten up the living room and wash dishes. Parents will be so grateful they might even tip you. At the very least, they'll call you again.

Pet owners can be just as wary as parents, so breaking into the pet-sitting field isn't simple, either. (Imagine handing your house keys to a stranger and saying, "See you in four days.") As with baby-sitting, put the word out among people you know.

Be prepared for three-page lists covering everything from Fido's favorite walkies to the vet's cell-phone number. Don't laugh; for some people, their pets are their kids. To find out what to charge, call a few pet-sitting services.

I'm your handywoman

Finding an apartment-management job is tough but not impossible, and it's well worth the search. Resident managers can get everything from discounted digs to a free apartment and a salary.

I'm in an ideal situation: The building I manage has only 21 units and relatively little turnover. In return for my rent credit, I keep an eye on the property, answer tenants' questions, show vacancies, run credit reports, get keys made, replace hallway lights and pick up trash. I'm on call in case a tenant gets locked out, but that hardly ever happens. However, my rent credit happens every month.

Resident-manager jobs appear regularly in the help-wanted ads. Some companies want experienced handymen. Others simply ask you to arrange for such work to be done. But all resident managers need flexibility and common sense, and the ability to get along with everyone, even annoying tenants. Especially annoying tenants.

Plenty of other occasional gigs exist, of course. You could sell blood plasma, mow lawns, clean houses, deliver newspapers or recycle aluminum cans.

Some folks have luck selling unwanted items online. It all comes down to what you're willing to do. If you don't mind fishing soda cans out of the trash, there's money to be made. If you're able to peel fresh hairballs off a carpet, you may have a bright future in pet-sitting. Find a job that matches your sensibilities.

Personally, I'm unfazed by having my blood drawn, calming crying babies and walking someone else's dog. And I'll probably never give up mystery shopping. How else could I get paid to marvel at $27 underpants?