Kathleen Lim wanted to move up the learning curve. At 24, she had just been promoted to billing operations manager at Box.net, a $10.7 million cloud-computing company in Palo Alto, California. It would be her first management job at her first postcollege employer. Box.net also wanted her to strengthen its collections department, not previously a focus. "I was looking for guidance, from best practices to how to structure my team," says Lim. "And there was this curiosity about what else is out there. How do other organizations do it?"

Greg Strickland, vice president of business operations, didn't know just the person to help her—but he did know just the company. An admirer of the collections operation at Salesforce.com, Strickland used LinkedIn to approach several people at the $1.7 billion company. "I said, 'We look to you guys as a model. Would you be interested in meeting with her on an as-needed basis?' " says Strickland. One e-mail later, Lim had a mentor: Hoang Le, Americas collection manager at Salesforce.

Mentoring is the concierge service of employee development: personal and customized, more or less on demand. But an in-house mentor isn't always easy to come by. Young companies may not have enough seasoned employees to go around. Even businesses stuffed with veterans may lack expertise in specific areas.

An external mentorship, in which a Luke Skywalker from one company finds his Obi-Wan at another, addresses both problems. "CEOs don't have to think, Whom can I hire?" says Kim Wise, founder and CEO of Mentor Resources, a San Francisco company whose software helps clients manage mentoring programs. "Instead, they go out and match their people up."

Box.net was a typical underpopulated start-up when it launched in 2005. Strickland estimates that at least half the employees sought outside guidance in those early years. With a staff of 265, the company now has lots of potential internal mentors—but also lots of folks with networks branching into other companies. When someone embarks on a project for which internal best practices don't exist—as Lim did with collections—executives like Strickland shake those networks to see who falls out. "One of the virtues of being in Silicon Valley is that there's so much knowledge outside of our organization," says Box.net's CEO, Aaron Levie. "We try to draw on the collective wisdom of our industry to help our employees be as successful as possible."

Levie says external mentorships are a good fit for Silicon Valley and other industry clusters (for example, finance in New York City and entertainment in Los Angeles) in which there exists what he calls a "culture of reciprocity." Big companies advise small companies. CEOs share best practices at networking events. Such cross-fertilization benefits businesses by strengthening links with potential partners, and entire industries by raising everyone's game. Pushing those relationships down to the level of individual employees is the next logical step.

Mentors, for their part, enjoy sharing knowledge and helping others. And, of course, it's flattering just to be asked. Wise estimates that some 65 percent of those approached say yes. Le, a 20-year collections veteran who had mentored interns at Salesforce, is typical. "I like to see people take my ideas and make them their own," he says.

Internal mentorships are meant to develop multiple skills and leadership talents over time; their external counterparts are more focused and come with an expiration date. But partners with good rapport can wander farther afield. Le has advised Lim, who has three direct reports, on how to structure her team and which tasks to outsource. They have discussed when to stop giving credit to a customer and how long to let invoices go past due. But Le has also shared his philosophy on performance reviews, feedback, and motivating staff. To bolster Lim's development, he has pointed her toward conferences and industry associations. Lim has questioned Le about his company's sales strategy.

Lim and Le have yet to meet face to face. Their first session, in August 2010, was an hourlong phone call; they now speak by phone roughly once a quarter. In addition, Lim e-mails Le with questions, and Le forwards her useful articles. Although Wise says such remote mentorships are not uncommon, she prefers some live interaction. "They can meet at lunch or for coffee in the morning," she says. "We've had mentoring pairs get together for Sunday dinners with each other's families."

External mentorships aren't meant to be long-term commitments: Wise says high-level mentors may balk at gigs lasting longer than six months. Fortunately, employees—including top leaders—can benefit just by shadowing someone from another business for a day.

Employees of the New York Jets had time for that last spring, courtesy of the NFL player lockout. General manager Mike Tannenbaum urged staff members to spend one day at an organization they found interesting to see what they could learn. Thirty did so, including Tannenbaum, who sounded out SAP CEO Bill McDermott on the software giant's people practices. Assistant coach Mike Devlin visited—appropriately—Coach, where a senior vice president explained the handbag purveyor's approach to bringing new employees on board. Employees chose the skills they hoped to develop and the mentors to help develop them. The sole requirement: Stretch beyond your comfort zone.

Lauren Reed, Tannenbaum's assistant, wanted to study someone working for a high-profile person in a fast-paced business. Her guide-for-a-day: Gary Dell'Abate, producer of The Howard Stern Show. A longtime Stern fan, Reed had noticed "how Gary is able to anticipate a lot of things," she says. "As an executive assistant, it's a skill you really need." In Dell'Abate's office, Reed observed that he kept the show playing low in the background; he carried on a conversation yet was poised to spring into action whenever the proceedings suggested he might be needed.

Anticipating the boss's needs is nothing new for Reed, who has been an executive assistant before. But "working for the NFL is very new to me, and I've spent a lot of time this past year just learning the business," she says. "I needed to re-hone my skills. It helped seeing someone else in another environment. I feel like I've been rebooted."

Go to www.inc.com/mentoring for more articles on mentorship.