Else you can, take cyanide, hang on to railings of a tall
building suspended in mid-air with one hand, the other hand you aim
a large caliber pistol to your temple. Once you open fire, you
lose consciousness involuntarily letting go of the grip on the
railing, plunging down.
Guaranteed results, if the poison doesn't get you, the bullet to
the head will, if both fails you, you will still plunge
to your death.