I dont like to get into battles (especially internet battling) to much but i'll give some insight

Punclines should always be clever and original like when Wayne said
"You might wanna fall back like August
or late September Whaterva you call it
I hit niggas in the head like Vondage"

or

Joe Budden
"Son I'm that nigga
gunz blast quicker
you in the pursuit of happiness
bum ass nigga"

"Your mans aint got no time to warn you
aside from that hoody there aint no nines on you"

clever and original not repetitive like a line about myspace.

Getting personal works sometimes Like Nas vs Jay, but i depends on who your goin against cause it could turn out like Ja Rule vs Eminem.

Current Events make the best punchline cause their new and draw attention:

Examples:

Talib Kweli

I'm prayin to the lord and askin for forgiveness
Just for kicking Niggas out of the club like Micheal Richards"

Joe Budden

"them hammers unload quick
my new shit is home invasion
find your wifey like Anna Nicole smith"

Thats my little two cents, if i get to in dept i would post a whole tutorial, but their is a logic to everything.