Japanese strategy is somewhat complicated. You have enough push to do one of several options, some of which overlap. You *must* keep an eye on what the United States is doing. Make no mistake, once late 41/42 rolls around, they are likely to go into the war. If they make their highest priority crushing you, they will. But of course, that virtually guarantees they throw away Europe, but you have to play with that in mind.
As Japan then, I have 4 "major" objectives, of things I want to do by the end of the game, and several minor objectives, which are incremental steps towards fulfilling the major objectives. Usually I can find I can accomplish about 1 and a half of my objectives.
The four major objectives then are.
A) Keep the U.S. from killing you, and securing vital supply lines.
B) Conquer China.
C) Hit the Soviet Union, trying to push them as far back as you can to support the Germans.
D) Attack the Commonwealth possessions in the Asia/Pacific, usually Myanmar, Burma, Singapore, and if you have an opening, India and/or Australia.
China is probably the most immediate of the four. After all, you start at war with the Chinese. Now, at the *very very* start, your position in China is strong, but not overwhelming. Chances are, your group is playing with Attack weakness, so the Nationalists can't counterattack all that well. However, the Chinese have a nice stack of 19 corps ready to slow you down. You have air supremacy, and better troops, but you don't have a *lot* of bodies, and your tac bombers are all awful, so the air power means less than it could.
So right off the bat you should decide what you want out of China. Almost everyone will want to grab the resources nearby to your lines, taking Sian, and the ones near Chang-Sha and Kwei-Yang, and generally driving the Chinese forces back to the mountains. That's not that hard, really. The decision you have to make is whether you really want to conquer China, which involves taking Lan-Chow, Chungking, Kunming, and Chengdu, none of which are soft targets. YOu'll need a *ton* of land units, probably an o-chit or two, and generally taking build points away from what you'd need for your naval budget. It doesn't get you a huge amount of production either; but it does free up an enormous army that doesn't need to oppose the Chinese.
Build needs. Lots of inf, bombers.
The Soviet Strategy.
This one is simple, sit down until the Germans attack, building fast things, your arm, mech, cav, even your mots. WHen Germany declares war, you follow suit, and push west as far as you can. The Soviets will probably force a peace when you take Vladivostok, so when they do, pile units on the border to get your garrison limit up, and at the very least force the Soviets to leave garrisons behind. You probably can't do this *and* any of the others.
Defending yourself/ British possessions. (They kind of run together)
To be honest, I think this is the one closest to what you were asking at the beginning. Your most immediate strategic pressures are a lack of oil. You've been getting 4 from the NEI and the U.S., but they go away once you're at war. You might have built one or two synth plants, but they're really not enough. So your most critical targets on that surprise impulse are the NEI and the Phillipenes, the latter being a dagger pointed at your heart. Your marines should be taking those areas, and division invasions should be looked at for Raboul, Pago Pago, and possibly Dutch Harbor. If you have some time for finesse, you can muster your faster infantry in Sawng Kla, and make a dash for Singapore during the surprise impulse.
Given the enormous number of things you must do during the surprise impulse, have an O-Chit saved up so you can play a supercombined. Lastly, you'll want to take a swat at Pearl Harbor. He's going to have at least one real carrier based there, vulnerable. Two if he turned the Bearn into a TRS or didn't intern it.
Ok, so you did your massive surprise attack. Let's say you're otherwise going the "historical" route. You've pushed the Chinese to the mountains, and you are leaving the Soviets alone. You'll probably need 10-12 corpst in China to keep the Chinese from reversing your gains. You don't need good units, especially on the Nationalist front or on the coast, but you do need bodies to stop off the Amoeba attack. (Where they just slide around your lines.) Mil, weak inf, and Territorials are good for this. If you're playing with the Chinese Warlords, that'll give you another guy or two to bolster your lines, but the Shanghai Warlord is useless, and the Peking one can only really defend hex 0637. I actually don't like to use Gar, since I like to put them on the islands. Having a move of at least 2 allows some mobility in China without flipping.
You'll need 2-3 units on the Home Islands to stop desperado attacks. (I lost the game this way once. Humiliating.)
Aside from that, the Pacific is too big to guard everything. You *must* hold onto Batavia (NEI oil) Manila (The Philipenes sea control) Truk, (More Sea control) Truk should have 2 Gar or the like on it. Or maybe even a Gar and a HQ, since Truk is a great naval base. Batavia and Manila can probably get away with a single corps garrisoning early, but later on you'll want 2.
A lot of people I know like to also stick guys in Kwajalein. Bear in mind though, that unlike the others, which you really want to hold, Kwajalein is an outpost. It'll fall, you just want to make them bleed a bit for it.
In the meantime, you'll want two "invasion stacks" comprised of a Marine on a TRS, and some other inf on an AMPH. They don't need to be actually attacking, but enough to threaten to hop behind any American advance, forcing him to slow down. They can also be used to support ambitions in India or Australia, if you have some mobile units to support them.
But in the Pacific, land units are actually the sideshow. You do most of your fighting with air power. (And carriers.) Speaking of carriers, you'll have at the *very* least the Kaga, Akagi, Soryo, Hiryu, Shokaku, Zuikaku, and the Ryujo. You also might want to build the Hiyo and Junyo, and possibly the light carriers, especially if you're playing with the Shipyards. (Build that carrier shipyard on the first turn if you're going for this, it saves a BUNDLE)
To oppose you, you'll have the 7 U.S. carriers, minus whichever one you smashed at Pearl Harbor, and the possibility of some CW carriers. ( A lot of CW players don't bother, since their carrier planes tend to be hard to fit on their carriers.) The Essexes can't come online until S/O 42. Until then you'll have a large advantage in carrier power. (Your planes are better at the start) Afterwards, you'll have to supplement your force with Land based air. My usual strategy is to build fenceposts; put some zeroes and Bettys in Kwajalein and Truk (and then when they fall, Manila and Merado), and your fleet somewhere behind the "lines". Don't come out until the U.S. commits, and have an HQ nearby to reorganize your carriers. Once you've seen where they're going this turn, either attack the fleet with LBA and your carriers if the odds look good, or if they don't, try to run around with your invasion stacks and hit behind him. Interceptions should mostly be used when his raiders come to chew on your CP, and later in the war, 43ish is when I usually start, you should be building your submarines to do the same to him, as his supply lines extend.
With whatever reserves you have, attack somewhere the CW wouldn't like to lose, like India or Burma. Further west means further away from the Americans. Above all, keep your carrier fleet intact as long as you can. As long as the 6 biggies are together, he has to step carefully.
And a cookie to anyone who bothers to read through my long rambling incoherence