If you want your organs to be harvested, you need to die within a few minutes of access to health care professionals. The two dependable options I can see are to call an ambulance before killing yourself or to kill yourself at a hospital (maybe the parking lot?) The problem is that they will try to revive you, and that would suck if you gather to courage to die and others prevent it. I would suggest to just do it away from people to assure that you succeed and scrap the idea of organ donation. I don't know your motivation for dying, but for me, I'm not going to take a chance of interfering with my plan by making a last effort to benefit others, when I don't know if it will actually benefit others.