Ambush
Usually, one main OP and numerous secondary OPs were established. Runners and radios were used to communicate between the OPs and the main command post. The OPs were located so that enemy movement into the ambush could be observed. They would remain in position throughout the ambush to report routes of reinforcement and withdrawal by the enemy, as well as his manoeuvre options. Frequently the OPs were reinforced to squad size and served as flank security. The command post was situated in a central location, frequently on terrain which afforded it a vantage point overlooking the ambush site.

Recon methods: Reconnaissance elements observing a potential ambush target on the move generally stayed 300–500 meters away. A "leapfrogging" recon technique can be used. Surveillance units were echeloned one behind the other. As the enemy drew close to the first, it fell back behind the last recon team, leaving an advance group in its place. This one in turn fell back as the enemy again closed the gap, and the cycle rotated. This method helped keep the enemy under continuous observation from a variety of vantage points, and allowed the recon groups to cover one another.

See also: Ambush predator
Viet Cong and PAVN battle tactics
Flanking maneuver
Flypaper theory (strategy)
List of military tactics
Sniper

References: 
Extract from Lt Col Anthony B. Herbert's Soldier's Handbook