Use Wooded Terrain to Enhance Cover & Concealment – when available use wooded terrain to provide both cover and concealment for your units.
Historical Example: The following image is from a US Army Field Manual, however it was first shown in a pre-war German manual.
Technique: place your unit (armor, gun, crew served weapon, or infantry) far enough into the woods that they can see out and not so far that they are blind. This will provide some cover from incoming rounds if your unit is spotted, but it will also make spotting them it more difficult.
Example: from my CMBN BETA AAR my Pz-IV was spotted by an enemy M10 sitting in the open on a hill. The panzer and the TD fired at roughly the same time… the enemy round impacted the trees, my round hit and destroyed the enemy M10. In this case the trees provided additional cover and saved my tank.