Keep their number in your phone. Hopefully you'll never need it, but IMO it's the second most important number after 911. Even if you have had a little bit of training, helping a suicidal person can be very bewildering and difficult, so if you find yourself unable to act effectively, you can pull out your phone, dial this number, and get help from an expert. I know people who are alive because somebody else (in one case a complete stranger) had this number in their phone:

1-800-273-TALK (1-800-273-8255)

If you find yourself dealing with a suicidal person and you don't have this number handy, dial 911.