The more planning you put into your cold calls, the easier and more successful they will be. Prior planning will take up some of your time, but once you've laid out your calling plan you will be amortizing that time with each call. It won't be long before your time investment pays off!
The first step in planning out your cold calls is to set specific goals. For most salespeople, one of those goals is to get an appointment with the prospect. However, it's not a bad idea to take a step back from that. Set your first goal as getting the prospect's permission to have a conversation with them. Your second goal can be getting the appointment, and a third goal might be getting permission to contact them at a later date (this is the contingency goal if you fail to achieve #1 and #2).
Once you've set your goals, the next step is to lay out a conversation flow chart. This doesn't have to be an actual script, although that's certainly an option. The conversation flow chart is a way of mapping out each of your goals, and the general path you're going to take to achieve each goal.
For example, if your first goal is to get the prospect to have a conversation with you, you might write out the exact opener you intend to use. You can also jot down a few responses you'd make to the most common objections you're likely to hear. Then you can chart out a few paths to take towards goal #2: getting the appointment. These might be more objection-handling responses or a few techniques to build fast rapport. Finally, you could write out a phrase or two for accomplishing goal #3.
Of course, you won't always reach a live human being on the other end of the line. That's why writing out a voicemail script can help, so your mind doesn't go blank at the sound of the beep. Knowing exactly what you'll say to the machine takes some of the stress off of you while you're on the spot.
Finally, before you make a call it's a good idea to do a quick bit of research on your prospect. This doesn't have to be a major effort. Even a minute or two of internet digging can turn up amazingly useful information. Remember, Google is your friend. So are Facebook, Twitter, and (for B2B salespeople) LinkedIn. Just plug in the prospect's name and see what comes up.