There's no greater gift than the gift of life, whether it be donating blood to your local Red Cross blood bank or registering to become an organ and tissue donor.
The following sites have great information on the process. Your organs won't do you any good in heaven, why not leave them here so a little piece of you can continue to live on?
If you're already a registered donor, make sure your family is aware of your decision. It will put them at ease to know your wishes if they are ever forced to make end of life decisions for you.