Our group has used a couple rules that have balanced the game very nicely:
Who ever controls Gibraltar controls the movement between Atlantic Ocean and the Mediterranean.
Who ever controls NW Europe controls movement into and out of the Baltic Sea
NO ONE can move ships through the Turkish straight into the Black Sea
*These all pertain to surface ships only. ie: Subs can still move through at will
Chinese infantry on any non-coastal territory defend on a 3
China counts their territories at the END of their turn to determine the number of men that may be placed.
If you can draw a line through India, Burma, Yunnan and Sikang at the END of the Chinese turn then China may place an additional infantry during their place new units phase.
When playing with these four rules we find the game to be extremely balanced and no single side wins more often than the other. This is with a player pool of roughly 10 players. We have others join us on occasion but we have ten regulars. Any thoughts from anyone?
I should mention that we also play with two additional rules on occasion, not every time but sometimes when we are feeling like spicing things up. These following two rules are not as well tested but have proven to be very fun for our games.
For every Artillery that is paired with an infantry on a coastal territory that is defending against an amphibious attack the artillery may fire in the opening fire step causing casualties before the enemy even makes it to the beach
AA Guns may fire to shoot down Rocket strikes (and yes radar will affect this)
I also wanted to note that we play with the most current rules for strategic bombing. This means that fighters can participate as escorts and interceptors. We have found this makes the air war a lot more dynamic. It also encourages fighting over Norway significantly more as the Allies can use this as a fighter base to escort bombers into Berlin.