Anything that isn't bolted down - loose the spare tire (replace with tire fix in a can). Loose the tools and jack as well. Get rid of any crap floating around in the boot (trunk). Clean out your glove box/door cards. Check under the back seat for anything that could be hanging around in there (I got dibs on the spare change!).
Before you start on anything adventurous (carbon fibre this, holes in that etc.) clean out all the stuff in the car - you'd be surprised how much weight all those CDs, magazines, street maps of Timbuktu and so on all add up to.