During her first job out of college, Ally Sperber never went in search of a mentor. Instead, Mary Ellen, one of the managing partners in the financial firm where both women worked, found her.

In Sperber, Ellen saw a kindred spirit. And despite their nearly 15-year age difference, the mentor-mentee relationship quickly blossomed into a close friendship.

"Early on, she embraced me and really showed me the ropes," explained Sperber, 27, who now works at a public relations firm in Los Angeles. "She showed me basic stuff, from how to be a professional to how to prepare for meetings. And when it came time for my next job, she even taught me how to negotiate my pay."

A good mentor can play an important role at any stage in a woman's career. But it's especially important during the first few years on a job.

"Time is of the essence," cautioned Victoria Pynchon, a co-founder of She Negotiates, a consulting firm that empowers women to stipulate equitable pay. "There's a very narrow window of time for women to be pulled into existing networks, especially inside corporate America."

Christine Silva, a research director at Catalyst, a nonprofit research group that focuses on women in business, also urged women to identify mentors as early in their careers as possible.

Silva also advises women to seek out a sponsor, whose role is different from a mentor's. While a mentor can help an employee navigate the nuances of office politics, a sponsor is someone who has her back when it comes time for a promotion.

Sponsors "have to be senior enough to have a spot at the decision table -- but they don't necessarily have to be a woman," says Silva. The mentor doesn't need to be female, either. "It's about having a variety of people in your corner, regardless of their gender."

"Don't pick a mentor based on their gender," Pynchon agreed. "Pick someone because of who they are." She encourages women to develop their own personal "board of directors": "You should align yourself with not only who has the power, but who has the courage of their convictions and who tells the truth."

In busy offices, many women are reticent about asking would-be mentors for guidance. Lisa Maatz, director of public policy and government relations at the American Association of University Women, tells women to be upfront and clear about what they're asking.

"Don't just ask someone, 'Will you mentor me?'" said Maatz. "If you’re asking someone to be generous with their time, realize the commitment might sound intimidating. Be clear that it doesn't require lunch every week or an endless chain of email."

Maatz also reminds mentees that they aren't the only beneficiaries of mentoring relationships; it's a two-way street. "When you do it right, it's the kind of thing that comes back to you ten-fold," says Maatz, who is currently mentoring a young woman in her office. For the mentee, she said, "The goal is to create an army of past and present coworkers who care about what you've done and believe in your work."

Here are four tips for finding (and keeping) a great mentor:

1. TIME IS OF THE ESSENCE. The search for a mentor should begin right away. Don't put it on your future to-do list for a year or two from now. Especially when starting a new job, strike while the iron is hot.

2. SECURE BOTH A MENTOR AND A SPONSOR. Look for support at varying levels of your company's hierarchy. Seek out a mentor to help navigate daily concerns and a sponsor willing to sing your praises when it comes time for a raise or promotion.

3. DON'T BE AFRAID TO ASK. If you wait for a mentor to seek you out, you could wait a long time. Remember, the worst your prospective mentor can say is no. When you ask, be clear and up front with what's required in terms of time and commitment. And don't limit yourself to only women. Men make great mentors and great sponsors, too.

4. BE ASSERTIVE. If you want a strong and assertive mentor, start acting that way yourself. Stride into a room. Make eye contact. Use a firm handshake. Choose the best available seat. Stop apologizing. And then, when the time comes, say thank you.