How many times have you tried to reach a client, co-worker, or business partner only to get sent straight to their voicemail? How many calls have you let go to voicemail? In our busy lives, we may spend more time listening to electronic recordings of voices rather than the voices themselves. The message you've recorded to greet callers is oftentimes your first chance to make an impression, so it's important that it's a good one.

I'm often traveling to and from book signings, media appearances, and public speaking gigs, so more calls get sent to voicemail than I'd like. When people can't reach me on the phone they hear, "Hello, you've reached Janine (pause) Driver. Please leave your name, phone number, and a detailed message so I can get back to you and give you my undivided attention."  
This message accomplishes a few goals. First of all, the pause between the first and last name ensures callers understand both, and it also signals to people that you're someone important. Try it—say your name with a two- or three-second pause in between your first and last names. It makes it sounds like your name appears somewhere in lights! It's simple, but it works. Once when I introduced myself this way to a client, enunciating both names and pausing ("My name is Ja-nine (pause, pause) Dri-ver") the client said, "Wow! Yeah I guess you are Janine Driver!" If you have trouble waiting the full time or if it feels awkward to you, try saying your middle name in your head while you pause. Remember, when we're nervous we often increase our rate of speech, so practice with a slightly longer pause than you'll use on your voicemail message or when introducing yourself.
Secondly, I emphasize that I want to give the caller my full and undivided attention. When you do this it's as though you're saying to them, "It's not that you're not important to me, it's that you're so important to me that I want to wait for a time when I can fully focus on your concerns."
Thirdly, I didn't apologize. You don't need to apologize for being busy with work. If you do, it may come off as insincere and it can even make you seem weak. If not apologizing worries you, just provide the caller with other options to get in touch with you instead (just not so many that you overwhelm them).
Feel free to copy my voicemail greeting, or pick from any of these other examples (and leave your own suggestions in the comments section!):