This post by ASL_Veteran has been literally taken from the forum. The original is here .
Playing good defense requires a great deal of skill and experience. I think it's much more difficult than playing offense. First you have to know your fields of fire. Second you have to keep your defensive forces repositioning - almost constantly. Third, you have to keep the attacker offbalance. You keep the attacker off balance by keeping some troops hidden who have a LOS to a kill zone and leave other troops to fire on the kill zone. Once the attacker has identified and positioned himself to engage the troops actively firing into the killzone, you reposition those and open up with the previously hidden troops. This way you constantly keep your killzones under fire and you are repositioning troops before the enemy can bring overpowering fire to bear.
I generally think of my AT weapons as the 'skeleton' of the defense. Because enemy tanks are the biggest threat to your defense, your defensive dispositions have to be able to deal with them first and foremost. So the entire defense has to revolve around your AT assets.
Second your heavy anti infantry weapons get deployed. HMG and mortar teams etc. These weapons are a little more difficult to redeploy when it gets too hot so they have to be sited with more care.
Last your infantry squads get deployed. They are just there to distract the enemy mostly and to cover areas where your heavy weapons can't cover. You also put them in the most exposed positions because they can be repositioned very easily.
Have back up defensive positions spotted for future use and don't hesitate to fall back to a second or third line of defense. If your squads are getting pinned by enemy fire it's (past) time to redeploy. It you keep your squads moving from position to position it also leaves lots of 'spotted' markers on the map so the attacker is never quite sure where your defenders are and that cuts down on area fire. Constant repositioning also reduces the effects of enemy artillery.
Sometimes I'll even keep smallish forward forces to engage an advancing enemy and I'll keep most of my forces back. Once I've determined the main avenue of attack I'll send my reserve forces to concentrate there.
The main rule of thumb for me is 'fire superiority'. If I don't have it I reposition. If I do have it I stay until the attack is defeated. The more the enemy outnumbers me the more I have to reposition. An aggressive attacker can frequently be lured into deadly ambushes. A cautious attacker will become even more cautious and chew up more time due to his uncertainty as to the composition of your defenses. In the case of a cautious attacker it may eventually be possible to hold up his entire advance with a couple of rifle squads while the rest of your force sits around eating K rations.