Depends on which bit(s) are doing the rattling. If it's the number plate (mine at the moment), a few well placed rubber spacers behind it. If it's major panels that are having a bit of a shake then look at sound deadening material. If it's just a few random bits and pieces then pad it out with rubber to make sure it can't shake any more. If you've got an old tyre tube lying around cut that up and use it to stop **** squeaking.
Bite off more than you can chew, and chew like hell - Peter Brock (1945-2006)