if im playing japan, i often force the americans to come play with me. i take any non-essential forces (usually have some troops and transports to play with by j5 or 6) and go play in alaska or hawaii. ive even gone over and taken brazil (that was a wierd game, had a jap IC in brazil). your goal is not to actually keep anything, just threaten america. anything america sends at japan is stuff not being sent at the germans. and like mentioned earlier, have the J help G in africa. G takes something, J reinforces it.
alot of folks dont have the axis powers helping eachother out much, each keeping to its own areas, but i think thats kinda weak. when playing the axis by myself, or with a certain good friend of mine, we usually wind up both powers in africa, and both in asia. this allows having reactive forces available, especially when it comes to pounding the poor brits.