Remind people to seek help, when someone is "joking" about ending it all. There is help out there, and it's worth an attempt, before doing something final and irreversible.
In many countries simply telling your doctor what you are considering will get you help.
If talking to a doctor might be a bad idea due to your profession or other reasons. Then talking with friends or family also might help.
I use Private Lock. It uses gyro to detect sudden movements and lock the phone based on that.