When you fix the rocks on the beach by preparing the SOS signal, the parachute drops every three game days. In the beginning you'll be happy to receive any old supplies. But towards the end of the game, when your collection is over 90% complete (or sooner), you may dread getting the same old stuff.
What I do, is to save the game during the night just before the supplies arrive. I check the supplies in the morning, and if they're not to my liking (because I'm after something more specific), I'll quit the game and then reload it - chances are you'll receive something different out of the 'chute.
Added by: jacamanda Nov 6th 2010, ID#14806