This might sound... harsh, but I would probably just tell them: "If you're going to kill yourself, either do it and get it over with, and don't and stop being such a little ***** about everything."
If someone wants to kill themselves, no one is going to stop them. They might change their mind after a failed attempt, but if they really think it'll make them feel better, or whatever... they're going to do it whether you want them to or not.
If it was someone I cared about, I would remind them that suicide may end all your problems, but they aren't the only person effected by it. Do they really want their family, friends, etc to go through the grief of losing you over whatever it is that made them want to die...
Either that or:
You know, it might take a lot of balls to pull that trigger (figuratively or literally), but we both know that that would be the easy way out. Prove you're better than that by facing whatever made you want to kill yourself.
Depending on who they were. I have actually said all three to a few suicidal people in the past. All of them are still alive today.