From: Leeuwarden, Netherlands
In addition to the above;
1) I build trains from the start and only in the Supreme HQ, and maybe a few other HQ's. Not the frontline HQ's. All things build and supply usually goes to the SHQ or a few others HQ's as the game develops and not to the frontline HQ's because that has the effect that their Exp. decreases fast. So a few trains in your SHQ is a great way to send new stuff to your local HQ's.
2) trucks same, a dozen or more in the SHQ, and a few in the local HQ's as well so your local HQ's can also Strategic Transfer some stuff. Be warned though that moving your trucks costs oil. The more you move, the more supply and oil you're using. Eventually it can grind down your army, as is what happened to the Germans quite a few times.... driving on gasfumes. So my choice in the beginning of the game is either foot or horses transport. Trust me, horses rule. Trucks and amoured transport is luxury.
3) The HQ you begin with, or the few HQ's you begin with, usually stay put, they become the SHQ's. That's where all the stuff and supply goes. The rest branches out from them, so you end up with a tree like structure (more or less). This all depends on open lines of comunication and supply lines... as soon as they are cut you're in trouble (as in you receive no supplies) and you need to set a few producing towns to deliver (if possible) their supply to the cut off HQ's.
4) yes you can build more small units to build a line, or retreat, *or* and that's my kind of playing... open the door, let your opponent see a weak spot and let him (or the AI) use it, make him send his troops into the gap that he thinks he can exploit and then spring the trap. Let him come and close the door, because you really don't want head to head bashing WWI style, you want encirclement and a sneaky attack in the back. Encircle, cut off, and bleed him dry. Having a few transport planes around to send supply to *your* cut off troops is a good idea too btw.
You'll notice that if you play against an AI that outproduces you and you try to stop it by building a solid line, all the AI does is build more and more untill finally it can break your wall. So you create a flexible line and let the AI come piecemeal to you, defeating his units a few at a time, thereby gaining Exp for your units as well.