I've played A&A very thoroughly over a number of years. I think by now I've tried everything, although one can never be sure. I finally lost interest in A&A when we proved an Allied strategy that would virtually guarantee an early and successful invasion of Germany. But at any rate....

Despite how pretty all the German units look at the beginning of the game, there really aren't enough of them to wipe Russia out. Even if you succeed, you'll be left nearly defenseless to England and the U.S.

Germany's defenses at the beginning of the game are really rather weak. It already has a lot of good offensive hardware, and it needs a lot of infantry cannon fodder so it can keep that hardware. It'll be a long, long time before other powers can buy and utilize the kind of hardware that Germany has at the beginning of the game. Economically speaking, Germany needs to maintain and protect this economic advantage, and not squander it on futile short-term attacks on big stacks of Russian infantry.

Consequently, we learned to play a very conservative Germany. Germany's main job is to stockpile units and keep from getting invaded. It's secondary job is to harass the allies, such as by running amok in Africa or the Middle East. A couple of tanks can tie up a lot of Allied units, as Russia and England will be forced to protect their underbellies. Very little of Germany's GNP should be spent on such efforts: accomplish the most with the least. Exploit the tactical holes in the enemy's naval defense. Buying lots of transports is silly. If you can't do it with one, you shouldn't bother doing it at all. Always ask yourself, "How much damage to the Allies' GNP am I doing with this move? How much GNP am I gaining/losing in territory and equipment purchases?"

Japan is the true expansion force, because early in the game it's really only got the U.S. to worry about, and the U.S. is several turns away from being able to do anything. Japan's job is to invade Russa. Once Russia is split into two fronts, its paltry infantry resistance will collapse. Then ***BOOM!*** Germany rolls over it with the stockpiled units.