Make it about anything ya want. Although cybernetic postmen probably wouldn't work too good for advertisement firms. :)
Mostly I need to be able to take 30 second snippets that have just a few words in them that sound cool enough to make people want to buy a product. The more generic the lyrics the better. ie, 'Zoom Zoom Zoom' for cars, 'Who let the dogs out' or whatever...
Basically just listen to any of the songs that are in the background of commercials for Coke, cars, running shoes, potato chips, airlines, mops etc...and think along those lines.
You need a catchy slogan, a kewl way of saying something like 'rock me' or 'fly me away' or just some way of running a few words together that sound good with the background music.

Put it this way..the money for licensing just one song to a major product line can get you 70 grand in licensing fees alone for a 30 second song, then there's royalties on top of that for every time it's played on tv/radio/theatres or anywhere else. (50% comes off the top for the publisher and then the rest is divided up among the writers)
If it's something that becomes a well known slogan (I'm lovin It - McDonalds), you can make a mint.

The recording quality on the vocal track has to be top quality. I've had several people send vocals that are really good but the recording quality or microphone was so bad that I can't present them to publishers. ie,

http://s93744050.onlinehome.us/mp3/Angels.mp3

...is a good example of a really nice vocal. With all the background noise on it though, I ended up having to add a bunch of sound effects (rain/thunder) to cover it up. This is fine if somebody is looking for exactly that effect, but otherwise most industry types want a 'finished product'. Or in other words, good enough so that they can take your recording and use it as-is.

So for instance, take some simple, 30 second pieces of background music like these and come up with something.

http://s93744050.onlinehome.us/mp3/Crystal.mp3
http://s93744050.onlinehome.us/mp3/Limbo.mp3

... or just take 30 or 60 seconds from a tune like this

http://s93744050.onlinehome.us/mp3/BushTube.mp3

Or you can take just parts of a tune and remove the drums and bass etc... to come up with something completely new...

http://s93744050.onlinehome.us/SchmangeOnline/Arrival_mp3_320/ArrivalNew.mp3
http://s93744050.onlinehome.us/SchmangeOnline/Arrival_mp3_320/ArrivalAtmosphere.mp3

Both the same tracks, but the second one has most of the percussion instruments removed and could be used as the background to a sci-fi film or whatever.