Sentry Removal

(Source)

... you may need to "take out" a guard or "good eyes." It's easy to sneak up to within about ten or twenty yards if you're good at stalking. At this distance a good bow and arrow is easy to hit with, and its silent.

If the guard is napping or dozing, you can get right up to him with ease. If so, a sharp knife can be used. Grab the guy, or girl, with your left hand over the mouth, from behind. At the same moment, draw your knife's sharp edge briskly, with a little pressure, against the right side of their exposed neck. Hold them snug for about ten seconds or until they stop kicking. This will sever the right jugular and silence is almost instantaneous.

If their neck is not vulnerable, grab them in the same way. As their neck arches backward, thrust the point of your blade to either side of the backbone and just above the hips. Quickly turn your knife 90 degrees and extract it. Repeat once more for good measure, but once is usually enough since the kidney is a fairly easy target, and quick.

--Joe Harold. Prepare Today--Survive Tomorrow, p. 158 (1980).

Update (11/2/2015): The advice for slicing the neck is not very good. See my more recent post, "A Pointed Discussion About Knife Wounds To The Throat."