I’m thinking more along the lines of:
“Come on, you apes! You wanta live forever?”
Than:
Here, drink this Kool-aide
Why?
So we can be with god on Hale-Bob Comet
:dubious: [CHIRPY] Okay! [/CHIRPY]
Follow the ball!
“He who keeps his life will lose it but he who loses it for my sake will live forever.”
King Leonidas had an inspirational plethora of these!
“The enemy outnumber us a paltry three to one. Good odds for any Greek.”
“Before this war is over, the world will know that few stood against many.”
“Remember this day, men, for it will be yours for all time!”
It was Flavor Aid.
It’s nothing personal, it is (or was) our job. I never gave it a second’s thought.
"Fell deeds await ! Now for wrath, now for ruin, and the red dawn! Ride for ruin and the world’s ending! DEATH! DEATH!! DEATH!!! "
“For Frodo!”
I know, it didn’t work for me either…
ETA: I much prefer Theoden’s one, up above.
I will tell you said that first, even if it kills me. 
I’ve got a $100 bill. All you gotta do for it is…
Henry V Act 3 Scene 1
Once more unto the breach, dear friends, once more;
Or close the wall up with our English dead.
…
I see you stand like greyhounds in the slips,
Straining upon the start. The game’s afoot:
Follow your spirit, and upon this charge
Cry ‘God for Harry, England, and Saint George!’
Henry V Act 4 Scene 3
…We few, we happy few, we band of brothers;
For he to-day that sheds his blood with me
Shall be my brother; be he ne’er so vile,
This day shall gentle his condition:
And gentlemen in England now a-bed
Shall think themselves accursed they were not here,
And hold their manhoods cheap whiles any speaks
That fought with us upon Saint Crispin’s day.
Si
I have my doubts that this speech, as rendered, would inspire anyone to die for the speaker. Even shouted from horseback, it would probably still lack a certain something:
“Hear me, brave soldiers! As you ride out to victory or to death, remember: IT’S NOTHING PERSONAL! It is, or was, OUR JOB! I never gave it A SECOND’S THOUGHT!”
At the very least, it couldn’t hurt to throw in some references to freedom or something.
Actually, it was phenobarbital mixed with vodka.
Going back to the OP-
The Heaven’s Gate suicide was said to have been done via Kool-Aid. Walloon was correcting that Flavor-Aid was used, but his cite indicated it was used at Jonestown. jjimm noted that Heaven’s Gate used a phenobarbitol/vodka mix.
If you fly the plane into the building, you get 72 virgins.
[Jack Bauer]We need you to do this. I’m sorry…there’s no other way. You have to or MILLIONS of PEOPLE will DIE[/JB]
“Die for me or I’ll kill your family.”
I didn’t say it was a quote or a GOOD things to say, but it would probably work.
“For you” or “With you?”
There’s a biiiiig difference there. The St. Crispin’s Day speech, along with the exortation on Omaha Beach were along the lines of “We die together facing fearful odds.”
My favorite would be the famous dwarfish battle cry “Today is a good day for somebody else to die!” 
Or a more-common variant these days, “die for us and you’ll be a community hero and your family will be financially rewarded.”
Actually, I’m pretty sure it wasn’t Leonidas who said that one, as it was the second-to-last line in the film, or so.
“We shall defend our island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender.”
“Let us therefore brace ourselves to our duties, and so bear ourselves that, if the British Empire and its Commonwealth last for a thousand years, men will still say, ‘This was their finest hour.’”
Both by Sir Winston Churchill