A good hook.....it needs to really sum up what it is that the song is about, in a few memorable and unique words. Or a single word, in some cases. All of your verses (and the music that acompanies) need to be written to lead up to the hook. It helps to set it up especialy well off of the line that precedes it. This will give the hook the extra power it needs to fulfill the listeners needs. I'm talking about a song that put's it's main hook at the end of the chorus. Of course, this is not every song, and there are tons of good songs written in other formats. Elton John, for example. "I'm a rocket man...rocket man, burning out the sootie pootie haww" or what ever he says. "Rocket Man" is the hook in this case. The last line of the chorus is almost uninteligible, but it works well as a sort of second hook.
The hook must be also be a constant within the song. What i mean is that if the lyrics involve a change of mind, a realisation of some kind, the hook needs to be something that still works by the end of the song. If he thinks he's a loser at the begining, and he'll never get the girl, but by the end finds his confidence to go and talk to her, the hook can't be "I'm a loser".
O.k, i hope at least some of that babel made sence. If this post was a song, you'd probably turn it off!