I was wondering this and got my answer quickly. So here's a quick answer for you. If you don't know. If you do, then click to another blog post.
The truth is, there are many more organ donors than people that need organs. A lot more. Like 10 to 1. The problem is that, most people's organs won't work as donations. The reason why is that as soon as the heart stops beating, tissue begins to die, and organs become unusable. This means heart, lungs, kidneys, eyes, skin, fingernails (just kidding, fingernails will keep and keep) and anything else that you can think of that is 'alive'. So car wreck fatalities don't produce usable organs.
The situations where you can actually use the organs and tissues are very few. Basically you have to be brain dead with your heart still beating, being kept alive by machines, and with no chance of recovering. Then your wishes ( often only if familiy is all right with it... see Living Will) may be taken into consideration, and your body is prepared for tissue and organ removal while your heart continues to beat. My understanding is that recipients have to be there and ready to receive many of the organs right after they're cut out of you.
In any case, the key here is that, even though you may want to be an organ donor, the chances of being able to be one are very slim. This is why there is such a need for organ donors. So that the tiny slice of people that can actually donate will include more people that expressed willingness to donate. For the record, I want to be an organ donor if it ever comes up.