The further away you are the longer it will take them to find you, they rarely travel very far if you wait for them to lose their alert status before going back to work. Stay far away on a hill in some bushes and they will almost never spot you. Even when the snipers were on alert with their laser sight dead center on my bush they wouldn't notice me.
having a stream/river between you and your target helps as well. enemies NEVER (for me at least) cross the river on foot and if there isnt a bridge (natural or man made) near by, that ensures a fair amount of safety. as i said though, thats been the case for my playthrough.