Asphyxiation.
While on the Moon.
At least you wouldn’t need to engineer a return trip.
Drop a safe on me.
Multiple organ harvest, for transplantation.
While in an induced coma.
Blowgun from orbit.
Blowjob in orbit
Yeah, this is pretty much me too, and in particular the “by surprise” part. I wanna be switched off like a light, and I don’t want to think about it while it’s about to happen.
It’d be ok if someone behind me in the courtroom did it a second or two after the judge read my sentence.
Put me in an open limo and drive me through Dealy Plaza. Let someone use a rifle identical to Oswald’s and blow my head off just where JFK got his. Film the whole thing at high speed and prove once and for all that my head would quite naturally go back and to the left.
Felix Baumgartner style jump with no parachute. I can’t think of where I would like to land yet, but I would want it to interrupt an important televised event just for kicks.
I was going to say Nitrogen asphyxiation, but for the greater good of the SDMB, I’ll opt for beheading by guillotine. Beforehand, we will establish a foolproof protocol that will enable me to signal whether I remain conscious.
I’m curious as to what this might be. Conceivably every muscle in your head will be paralyzed and immobile whether you’re conscious or not.
Oh, and I forgot to mention – thanks for this generous offer!
Sorry, that’s only for cartoon characters. 
Guillotine. Chop chop! Even if I remain conscious for a few seconds, that would actually be kinda cool – you know?
Yeah, but yours could have been a conspiracy too. Just the kind of thing the government would do, you know? 
Having an astronaut who didn’t have to come back to Earth would make a trip to Mars quite a lot easier. If I had to die, I might contact NASA . . .
The governor’s sworn not to pardon me, huh?
Okay, fine. I want to be clubbed to death–instant head pulverization. The business end of the club should be a 50-pound weight of lead inside the governor’s preserved skull.
Your move, guv.
Guillotine or firing squad. Short and to the point.
Why? The cranial nerves would not be severed.
If you HOLD your breath in a vacuum, your lungs will rupture.
Ask any SCUBA diver what the effect would be if you swam down to 30.5 feet depth, took a nice deep breath of air from your tanks, and then shot to the surface while holding your breath. <shudder>
On second thought . . . I’d love to be the first human on Mars. No return trip necessary.
Death by nuclear explosion. (I’m kind of torn if I’d rather be standing right next to the thing when it went off, or just far enough away that I could see the flash, but close enough to still be quickly ripped apart by the shockwave)
Realistic choice: Firing squad. Standing, no blindfold.
Does the manner really matter? I guess I would want it short and quick and painless but the main issue is they gonna kill me.