It's your own decision what happens to the organs and tissue in your body when you die, but I have always been a supporter of donating them upon death. Rather than waste them, it's better to give them to someone who needs them. There are a lot of people dying because they can't get the organs they need to live.
My dad was dying of liver failure in 1996 and was suppose to have died on the day of his surgery, but he managed to get the new liver he needed in time. A construction worker was killed on the job that day and he was a donor, so his liver went to my father. Unfortunately, there was another man at that hospital who also needed a liver...he was #2 on the patent list, meaning he had to wait for the next one to come along. That guy died.
It's a serious problem and happens everyday, but people rarely think about these things or know that it's even happening. It's not hard to become a donor and those body parts aren't going to be necessary in the afterlife(if you believe in one).
http://www.organdonor.gov/donor/index.htm (US site)
http://www.uktransplant.org.uk (UK site)
Those organs won't go to waste and will likely save someone's life somewhere. Heck...in a way, you could also "live on," so-to-speak.

I have a little card that I just have to carry in my wallet, and you can have it put on your drivers license.
KG