The ideal way to set up an informational interview is through a referral, where one of your contacts introduces you to someone he knows via email, and the two of you set up a meeting from there. Sometimes this will come about as the result of a casual networking conversation or drinks with a friend, but you can also be proactive about setting up informational interviews. Identify the top companies you want to work for and target people at those companies for potential meetings. LinkedIn is a great tool for figuring out who you know that could refer you to someone at a company you're interested in. Because they allow for someone to vouch for you, you'll have the most luck with setting up informational interviews via referrals, so use this strategy as much as possible.
However, you probably won’t want to ask the same contact to refer you to a ton of people (unless that person is more of a friend than a contact), but you can build your network quickly by turning each meeting into another one. For instance, if you ask a question in the informational that someone in another department might have a better answer to, ask for an introduction when you send your thank you email. Or, if you really hit it off with the person, you can ask if he knows anyone else you should be meeting with. You’ll build a long list of contacts if you can keep up this pattern. But be cautious not to ask for the next meeting until you've built a solid rapport -- no one wants to feel used or mined for their Rolodex.
What if you don't know anyone with a connection to someone at your dream company? There's no harm in sending a cold email -- the worst thing that could happen is that you get ignored and never have the meeting, which is the exact same outcome as if you had not reached out at all. See if there's anyone you can email that you have an organic connection to, even if it's thin -- maybe you share an alma mater or are both members of a certain professional organization. If that’s a challenge, you can always email someone blindly and hope that they write back. As long as you're professional and courteous, there's nothing to be afraid of!
One final thing to remember -- put the other person's priorities ahead of your own. Shift your schedule so you’ll be able to meet them at their preferred location and time. The last thing you want to do in setting up an informational interview is to pose any inconvenience to the other person – after all, they’re doing you a favor.