As of December, 2012, it is illegal in every state for a non-medical professional to assist someone else's suicide.
Every state has a slightly different definition of what it means to assist a suicide, and some states do not actually define it in their statutes. Generally, someone assists with a suicide when:
Note that it is not considered assisted suicide to fail to resuscitate someone who is already dying.
There are two major public policy reasons why assisted suicide is generally illegal. First, many states consider the preservation of life the highest priority. Consequently, in those states, anything that ends a human life is considered a crime such as murder or manslaughter.
The second policy reason against assisted suicide is that it can be difficult in some cases to distinguish it from murder or manslaughter. States have an interest in ensuring that no one is forced to commit suicide through threats or deception. For this reason, the three states that currently allow physician assisted suicide (Washington, Oregon, and Montana) have strict procedures that must be followed before any lethal medication is administered. They include ensuring that the patient is a resident of the state; and that at least two witnesses (some of whom have to be doctors, and unrelated to the patient) testify that the patient is mentally competent, that the patient is ill and predicted to live only a short while, and that the patient's request was voluntary.
For more information on physician assisted suicide see Is There a Constitutional Right to Physician-Assisted Suicide and to learn more about planning for end of life care, see FindLaw's section on living wills.