"We'd love to have a best-selling author write for us. We don't pay writers but you'd gain a lot of exposure." I receive invitations like that all the time.

I also receive invitations to speak for free. Sometimes organizations even expect me to pay for my own airfare -- which means it'd actually cost me money to speak at an event. While that business model may work for some people -- like those who sell products from the stage -- working for free usually doesn't pay off.

I have worked for free many times, however. But as an author and a speaker, I have to be careful about how much time I devote to unpaid work. Fortunately, I've gotten better at identifying when it makes sense to work without pay.

Here are four times working for free might make good business sense:

1. You'll gain legitimate exposure.

Whether it's a blogger looking for free content or an organization looking for a webinar for its employees, the word exposure gets used liberally. But not all exposure is created equal.

If your friend asks you to create his website for free in exchange for your name appearing in the fine print, will that really help you gain more clients? Well, it might if your friend is a world famous blogger who garners millions of views every month. But even then, the exposure you gain isn't likely to skyrocket your business.

If, however, your friend agrees to write a blog post about how your websites are different and why people should hire you, the exposure you'd gain could be worth it.

Before you agree to work for exposure, ask specific questions: How many people will your work be exposed to? How many target customers will be in that audience?

Keep in mind that, usually, the bigger the audience, the more money an organization or individual should have in the budget to pay you. So if they're refusing to pay you for your time, you might want to think twice.

2. You'll gain real-life experience.

Gaining experience could help you land a job. Whether it means taking pictures to build a portfolio for your photography business or speaking to audiences to gain paid speaking engagements down the road, working for free may be a productive step.

But it's important to set a time limit on how long or how much work you'll do for free. If you're still working for free after three years, you have a hobby -- not a business.

So ask yourself, "What do I hope to gain from this?" Have a clear goal in mind whenever you agree to volunteer your time.

It's also important to make sure you don't come across as desperate. Contacting strangers out of the blue to offer your services for free might backfire. Organizations may not take you seriously if you're not putting a price tag on your work.

3. You'll gain an impressive addition to your résumé.

Certain types of volunteer work can boost your résumé. If you're a new blogger, you might decide it's worth your time to submit free content to high profile sites. Being able to say you have written for the Huffington Post, for example, sounds better than saying you've only ever published content on your own blog.

So if an organization invites you to work for free, think about whether your efforts might help you build a future career. If it's a well-known company or individual, there's a chance you'll gain credibility that could help you land more lucrative jobs down the road.

4. It's a cause you believe in.

Of course, there are times when working for free is about giving, rather than gaining. So if there's a charity or organization that could use your assistance, helping out could be time well spent. Just make sure you don't expect it to catapult your career.

Set limits on how much volunteer work you do. Whether you decide you can donate one hour per week or you give one day per year, a concrete time limit can ensure you aren't overextending yourself. Then, when asked to do more work for free, you can say, "I've already reached my limit of how much volunteer work I can do."

Value Your Time

Many organizations make big promises about how working for free will help you. But most of those claims and promises just aren't true.

Don't get lured into something based on the hook that your work could "turn into something big." There's a good chance you'll end up donating a lot of your time and energy without getting anything in return.

If you don't value your time, other people won't either. So it's important to know your worth. And if you're going to do some work for free, be intentional about it. Make sure it's something that makes sense for your business.