If you're playing the Germans, I have one word of advice: Run!
While admittedly a little flippant, it's not totally wrong.
You desperately need defensive terrain, and you need to get the hell away from the Allied naval guns.
The west isn't too bad, lots of lovely bocage and as long as you blow up every bridge you can get your grubby little hands on, you can slow the Allied advance to a crawl.
Granted, Cherbourg will fall, it's only a matter of time. But you can certainly frustrate the Allied player as he attempts to get there.
East of Caen is unlikely to see much action. Too cramped with too many canals, and not a whole lot of VPs over there.
Which brings us to the center, Caen to Carentan.
Don't even attempt to hold a line here. It's fairly open farmland, you have very few forces there, and you're within range of Allied naval guns.
You cannot hold there, and trying to do so will only lose you forces you desperately need.
Use some low-value units (e.g Ost-battalions) to try and delay/annoy the Allies, but everything else falls back to Caen and the bocage-line, and starts digging like their life depends on it (which it does).
Speaking of terrain, don't be afraid to give it up, even Caen if need be.
Granted, Caen is nice to hold due to the road-network, but not at the expense of your army.
Force-preservation is much more important than some arbitrary point on the map.
Lost terrain can be retaken, lost units cannot.
And in campaign-games VP locations aren't that important. Caen is 4-500 VPs which is nothing in the grand scheme of things. The Allies will lose more than that just in the initial air-drop alone.
Finally, don't overlook the importance of fatigue. It is much more important in campaign games than in the scenarios where you pretty much have to push constantly.
Medium (yellow) fatigue will drop your combat effectiveness by 10%, high (red) by 20% and max fatigue by 40%.
But it gets worse, since each level of fatigue also drops your morale, further decreasing your combat-power.
Short version, once a unit hits yellow fatigue (100+), it's time to get it out of the line and replace it with a fresh one.