By Caroline M.L. Potter, Yahoo! HotJobs

Are you between jobs? At a career crossroads? Trying to transition to another field? An internship may be your best next step, even if you haven't seen the inside of a classroom in many years. No longer just for college students or recent graduates, internships can be a passport to success for professionals of any age.

You may feel sheepish about interning as an experienced professional, but you shouldn't.

"While internships used to be optional for a college student, they are now almost essential," says Rivka Kawano, an expert in the career search field and president of LifeTrain LLC. "And even experienced workers can gain great benefits from taking the time to do a part- or full-time internship."

To wit, in 2009, media company WowOWow.com had four interns who were all north of 40. Also that year, more than 50 of New York City's laid-off finance workers (each of whom had an average of 15 years' experience, according to The New York Times), took on 10-week, unpaid internships at startups in and around Manhattan.

If you're interested in interning but don't know where to start, read on to learn how to find an internship that's right for you and make the most of the opportunity.

Where to Look

Michelle Mercurio, a regional career services director with DeVry University and Keller Graduate School of Management, says finding an internship doesn't have to be hard work. "I would encourage more experienced candidates to ask their professional networking contacts about how to leverage their experience on short-term internships to keep current in their field," she says.

Kawano concurs. "Don't just look for internships that are being advertised," she says. "Find companies that have the culture, goals and values that resonate with who you are and where you are going. Take the initiative to call them and offer your services. In this type of environment, many companies would be happy for the extra help and are willing to invest in you."

Even if you've been out of school for a long time, your alma mater's career center may be able to help you pursue internships. Further, your alumni directory may also help you find folks at target organizations who are willing to help fellow graduates with their careers.

How to Sell Yourself

You're no longer a student, so don't pretend to be. Use your prior professional experience to your advantage. "If a job seeker is not a college student, it can seem awkward to be looking for an internship," Kawano says. "The key is to remember that you bring unique assets to the table that the younger intern does not."

Mercurio reminds workers that internships allow companies to evaluate potential candidates for future positions. "In many cases, employers are willing to take a chance on a motivated intern with strong transferable skills, but they won't take that same chance when they are hiring a full-time employee,"  she says. Internships often lead to permanent hires as employers have an opportunity to gauge one's performance and commitment in real time.

What to Expect

As soon as you start your internship, set clear expectations from the beginning, Mercurio says. "Make sure that both parties understand things like hours, length of time expected, what will be accomplished during the internship and what happens when you do get another job," she says.

Kawano believes in initially structuring an internship around a specific project instead of routine tasks. "Being able to say, 'Achieved fundraising goal of $100,000' sounds much more impressive than, 'Answered phones,'" she says. "This will also give a scope to your internship that will prove invaluable."

If It's Less Than You Expect

Sherri Elliott, author of Ties to Tattoos: Turning Generational Differences into a Competitive Advantage, doesn't think you should be too crestfallen if your internship doesn't lead to a job offer at the company. "If you intern at a company you don't wind up working for, you're still making contacts, staying on the market and opening up doors to the places you want to be later on," she says.

Kriss Poll of Introspective Advantage, a consulting firm that works with college students (undergrad and grad) and corporations around career development, urges interns to focus on the aspect of being productive, even if your internship is a disappointment.

"It is excellent for a person's morale to have a purpose that requires them to get up in the morning and leave the house," Poll says. "So many people are suffering from depression given the state of the economy and their own personal situations. You need to bring your 'A game' when you go to interviews. Depression makes that more difficult, but staying productive can counter depression."