Haha, good luck! The best way is to hold them up, and tell them to 'Get down'. They will stay face-down forever, until a base alert or somebody finds them.
Best strategy is tranq or strangle a guy, carry him somewhere quiet/secret, kick him to wake him up, and point your gun at him immediately. You'll immediately say 'Freeze' and the guy will turn onto his front. Then he'll stay there forever until alert etc. You can also interrogate him once he's there.
Edit: also as danowat says above, if an enemy finds any KOd/sleeping guard, they will wake them up. If you're clever you can place KOd guards and use them as a distraction to get even more guards
And remember if there are two guys within a few feet of each other, you can throw one and CQC-chain-throw the other one immediately, no alert if you do it right.