I can endorse the recommendation of bitumen. It really works!
You need to cut the sheets to size; they are self-adhesive, and I found that with a hair dryer you can mould them to the contours of the inside of the bodywork (I think this would be especially important in our cold winter weather). The doors are easiest to do, because the door card is so easy to remove. Do be careful about getting in behind the clear plastic with the sticky edge: that adhesive ticks to everything, and is hard to get off clothes, etc.
I also sandwiched adhesive backed acoustic insulation foam between the door card and the clear plastic.
I bought mine from Car Insulation UK:
http://www.carinsulation.co.uk/
Now the doors (which are heavier owing to the bitumen) shut with a confident 'clunk', and without reverberation. The stereo sounds better as well (I put new speakers in at the same time as doing the insulation).
A great advantage of bitumen is that you don't need to worry about moisture making the insulation wet and rotten. This is particularly important in the doors, with the electric windows going up and down.
See this thread:
http://www.igmaynard.co.uk/bongo/forum/ ... 47&t=60798
