when my last z was totalled, they tried to cut me a check for 6k (KBB value at the time) I simply wouldnt accept, and I made sure they knew it. I told them I wouldnt accept ANYTHING under 18k, as I had proof of other similarly equipped cars selling for that price. I finally settled on 10k and them giving me the car back for free (all aftermarket parts still in perfect shape)....
Documentation and presentation are key, Just keep in mind that EVERYTHING is open to negotiation, and you do not have to accept their offer.