I used sound insulation felt in the rear internal wheel arches. I just put as much as I could get in and I made sure that there weren't any voids which is very important, a layer in the boot aswell.
My brother bought the felt and he can't remember where from, it's like the stuff in the link below.
http://merlinmotorsport.co.uk/product_info.php?products_id=6072?utm_source=Google&utm_medium=Shopping
Everywhere under the rear seats and on the arches under the plastic trim I used at least one layer of dynamat, the dynamat is really just to stop resonance on thin metal panels but it's better than using nothing if there isn't enough room for the felt.
In hindsight it probably would've been better to lay one complete layer of the felt underneath the whole carpet from the front footwell to the boot and everywhere else if there's room, even under the rear seats too but at the time I wasn't sure if they would go back in with a layer of felt in the way ( I should've tried ).