O.k.,this is from my classes at BMI Nashville.
First off,if you write rock music,none of this applies.
and secondly,this is for "hit"songs,not real songs that actually MEAN something

!
It's all about "Her"
Write things that women like and can relate to.(Women make up a HUGE portion of the market that actually BUY music and go to concerts and call in requests.)
Write things that say things you'd say to someone in a conversation,BUT say it in a new and inventive way.(anyone can say The car was in the driveway..you have to say it in a way that makes the listener SEE it there.)
Don't bore us,get to the chrous.(this one's easy,you can't drag out 3 verses or have more than 8 lines before the CH or it's just too long)
If a listener can't sing the whole CH after the first listen or atleast hum it and sing a few words,then you need to fix it.
(This means it has to be uncomplicated AND catchy)
3:15 Easy,if it dosen't fall in this time frame,it's not going into heavy rotation on any station that has listeners.(this one actually does fit Rock as well,Clear Channel SUCKS!)
These are some I can remember off hand.My teacher was Jason Blume,he's done books and write all kinds of big hits and he runs the classes at BMI Nashville.
The Two BIGGEST things I can say are that you have to work hard on the conversational writing and then work on re-writing it in a fresh way(not like a George Straight song!!!!!,People only like him cause he's SUPPOSED to be great and country fans are LOYAL all the way.Plus he was in an even gayer movie than Brokeback Mountian called Pure Country)
AND,that you CANNOT be married to a song once it's "finished".They're not finished untill they're on the radio and everyone has heard it!
You can write and re-write it 100 times if the base of the song or the music is great.
Never throw anything out!
My teacher wrote his first hit 9 times.He told us he wouldn't be teaching this class or driving a new BMW convertible if he'd only re-written it 8 times!
I'm not an expert,but I listen to people that are so there ya go!
Oh yeah,like another post said before me(Patrick),if you hit a wall and it's not coming,leave it and come back to it.You could come up with an awesome verse at 4:30 am the next night and get up,write it down and leave it alone again.You can't force it,that was great advise.