Let say in purpose of efficienty :
1) group 1 ARM with 2 MOT or 1 MOT with 2 ARM, you will get a 5 % bonus when fighting, and still have good speed.
2) Use brigades only with the expensive troops (ARM MOT MEC), do not build ART. SP ART can be used with INF also, but even if efficient in most case of fighting (you will have enought ORG to move and conduct another operation), it's ressources unwise (for the same amount of ressources used to build and maintain your brigades you can have more INF), so that is a choice that depend on how you play : if you love the caterpilar strategy, mass build plain infanterie and no brigades, if you prefer to use well made strategy that will turn banana your oponements and imply a lot of micromanagement then build brigades.
3)Mass building militia, grouped by 12 under FM is another good way to go , and it will be infact a middle way btw the caterpillar strategy and the Von Manstein/Napoleon way to do the Art of War. Mostly due the fact militia are weak units you have to precisely choose your schwerpunk and plan carefully what you will do if shit happend.
4) The way the game handle figting (include naval), better to privelegy small units (corp level) backed up by a big unit. One army (9 to 12 units) backing up 3 / 4 corps is a brainless way to be sure nothing really bad can happend.
5) Be sure to alway have enought ressources before to mass build ARM, MOT or MEC, and be also careful to the TC. To much Pz divisionen will in fact kill you.
6) Keep the special forces for special dury under Comando leaders. Mount are great when used in hills or mountains but are worth nothing in plain and they can kill your manpower.
7) Always retreat/stop attack when you see Org going down too fast. you will save on the MP.