In Canada at least, you pretty much have to die in the hospital for your organs to be harvested. Most organ donors do not end up having their organs used.
However, you could consider donating your body for medical training or research. Usually you arrange this with a nearby university, and each one will have its own setup to get you on file as a donor. You can be both an organ donor and a body donor, though after you die, your body will only be used for one or the other.
If you kill yourself with something really poisonous like cyanide, they won't use it, since it can be dangerous even to handle your remains. However I think you would have a shot with the body donation route as long as it's something like traumatic injury or asphyxiation. I plan to use nitrogen, myself.