I'd take the (insert tool of death here) away from you and hold you until it ended; coaxing you and trying to calm you down.
And if that doesn't work; try and knock some sense into you (without hitting you physically)
I'd say you are being an attention whore and if you was going to do it you would have done it sooner. Or I would probably (If I believed you) tell you not to do it.
I'd tell you that everyone has tried but that I know you won't go through with it because your asking us to stop you which means you won't do it. So honestly I'd tell you to go ahead and do it.
If you were hanging yourself;
I'd ask you if you had any spare rope or cloth or whatever you'd use.
If you were taking some pills;
I'd ask for whatever's left.
If you were cutting your wrists;
I'd ask if I could use it after you.
If you jumped off a cliff;
I'd run over yelling at you to stop, then when I catch up to you before its to late I'll suggest that we jump off together. :) It'll add to the adrenaline rush before our untimely yet at the same time timely deaths. ^_^
If you were to move in front of a moving car;
I'd stand there right behind you (stronger impact by being moved by two exceedingly, extremely fast and furious massive (NO offense, not callin' you fat or anything) objects...
If you were to drink posion;
Ask or just take whatever's left (I mean, you'd be dead anywyas...it wouldn't really bother you if I took the remainders of whatever you used when you're dead, wouldn't it? =/)
Scream, 'STOP!' No ONE has the right to do suicide. Trust me, I've tried it plenty of times... not fun, at all. It's a serious matter and I wish people in places such as the media would understand just how tortured it can make people. Just NEVER poke fun of something like that with me, because it's not funny in the least!