Send a Bullet not a Man!

[Zemke]

coordinating units to bring in optimum firepower on your opponent in changing terrain is one of the major skills to learn in this game...

The key phrase in what DK said is "changing terrain". If you are going to use FOs to call in fires at the decisive place and time they have to be able to see the target area. This usually means some elavated terrain, and usually your opponent is not going to give up elavated terain without a fight, so keep that in mind. The next thing I have run into is LOS to the target. Lets say you do a half-ass map recon, and you pick out a hill for your key 155mm FO to go to. You have a fight, take some loses, but you take the hill, and then you find the hill DOES NOT have LOS to the target area you need because of intervening trees or builds or another hill. Lesson, ALWAYS go down to the lowest level and check all your potential LOS positions before the battle starts, if you are depending on your artillery, the FO cannot see, you have just made things very hard for yourself. Take the time at the start, plan your attack. If you leave it to luck and you do win, you were lucky.