Some companies and agencies have great programs in place that match you with a mentor based on specific criteria. I'm not saying you shouldn't take advantage of those programs -- you should -- but take some time to evaluate the qualities you're looking for. There's nothing wrong with having a second mentor on the side if you're not learning from the person you were paired with. If you don't have a mentor program, get out there and find one on your own.
Here are some questions you can ask yourself when you're on the hunt for your own personal guru:
Is this person doing what I hope to be doing one day?
While you can certainly learn from people outside your career path, you're more likely to be inspired by a person whose shoes you hope to fill.
Do I look up to this person and respect his or her work?
People can have awesome job titles and be terrible mentors. You don't want to end up with somebody who's exactly where you want to be in 10 years, but got there by sleeping with the boss. Make sure your mentor embodies the values you hold for yourself. My "mentor" (who probably doesn't even know I considered her that) was a very intelligent, strong, independent woman. I learned a lot from the work she produced, but also from the way she did it.
Will this person help me get to the next level?
You want to find someone who will give you advice and have your back, but more than that, they should respect you enough to give you challenging work and constantly push you to be better.
Your perfect mentor might not be obvious right away. I knew I had found mine after I'd been working for her for several months and realized I had accumulated a massive stockpile of her work in a file for future reference. She probably has no idea her work has become what I fondly refer to as my Marketing Bible. If you don't feel comfortable asking somebody to "mentor" you (I never did), let it happen organically. Go to them for advice, study their work and volunteer to help with their projects. You'll get out of it what you put into it, so I say go all in!