1. Don’t tell me you’re not home and/or unavailable.  I’ve already figured that out.  My keen powers of deduction enable me to distinguish between a recording of you and… you know, you.  Also, when you say you’re "unavailable", I take that to mean you are either on the toilet or having sex.  And maybe I don’t want that mental image, thank you very much.  But most of all,  I’m not sure I believe you when you say you’re "not home" or "unavailable".  You could be screening.  You know you do it, and I know you do it.  So let’s stop the lies.
2. Don’t tell me that I have "reached [your] answering machine".  No shit, Sherlock (see #1).
3. Don’t tell me to leave a message after the beep.  Just as you may or may not be home, I may or may not want to leave a message.  So what’s good for the goose is good for the gander — I’ll leave a message if I want to (you’re not the boss of me!).  And by the way?  If I do want to leave a message, I don’t need instructions.  Answering machines have been around for a while — did you seriously think I was going to start talking in the middle of your outgoing message?
4. Don’t tell me "you know what to do (at the beep)".  That’s so late 1980’s.  And it wasn’t amusing then.
5. Don’t be funny.  Chances are, you can’t do a worthwhile comedy shtick in under 10 seconds.  And even assuming that you have a funny message, it’s not going to be funny the second, third, or fourth time.  In fact, it will just become increasingly annoying.  Oh, and if you do that lame "Hello?  [pause]  Hey! How are you?  [pause] Hahaha.  Just kidding.  I’m not really here" bit, then I’m never calling you again.  Ever.
6. Don’t tell me you’ll get back to me as soon as you can.  We all know this is a lie when we say it, so we all know it’s a lie when we hear it.  And that’s fine.  I understand you’re busy.  So am I.  You’ll get back to when you feel like it, if you feel like it.  That’s totally cool — it’s understood as part of the social contract.  So don’t lie about it.
7. Don’t mention your pets as being unavailable to take my call.  Guess what?  I wasn’t calling to speak to them anyways.  You know why not?  Because they’re animals.  They can’t talk.
8. I don’t want to hear your baby gurgle and googoo.   Maybe if I reach you instead of the answering machine, and if the baby is up and in a good mood, I’ll indulge the aw-cute gabba-gabba.  But not on your machine, okay?
9. Nor do I want to hear Stephen Hawking.  Look, I’ve just made it easy for you by suggesting certain things you can omit from your outgoing message.  The very least you can do is have your outgoing message be in your own voice.
10. But just one of you is fine.  I don’t need to hear the voice of every household member.  This isn’t the closing credits of The Waltons — it’s an outgoing message. Besides, the image of you and your significant others huddling around the phone to record that message — well, it makes me throw up a little in my mouth.   So please, elect a spokesperson.  I’ll understand that he speaks for everyone.
11. Don’t play music.  Seriously, if I want to know what’s on your iPod playlist, I’ll go to your blog.
12. Don’t tell me to "have a nice day".  You don’t mean it, because for all you know, I’m calling to sell magazines or get you to switch to AT&T long distance.  It’s nice that you want to be nice to me — but being cheerful to everybody without knowing their identity doesn’t make you a "good person"; it makes you Forrest Gump.

This guy is right: short and sweet.  Say your (full) name, your phone number (just to be sure I got it right) and get off.  And if you have one of those answering machines or voicemail services that require fifteen seconds of dead air before it offers the beep, get a new one.