Here are some "house rules" that the group I'm part of has been evolving over the last 3 months. Always trying to refine/expand/contract them. Comments appreciated 
tech sharing
(after MUCH adjusting/tweaking/play-testing)
Every country may share tech once per month to any country they choose. The nations of Germany, UK, USA, USSR, and Japan may only receive "silver" techs. France, Italy, Nationalist China, and everyone else may receive "gold" or "silver" techs. No doctrines (land/sea/air) may be shared ever. AI tech sharing (obviously) has no bearing on these rules.
supply convoys
"neutral" supply of beachheads during conflict is forbidden.
alliance switching
We generally permit alliance switching (for example France joining the Commintern) although this has been under heavy debate recently. While it hasn't happened in our games, the USA "exploitation" into joining the Axis or Commintern does lead to a need for some kind of restriction
commonwealth force control
After a player who shall go unnamed, joined the Allies and immediately took control of the Canadian, South African, and Australian forces to engage in a Greek campaign against a neutral Turkey - military control of Commonwealth partners should be restricted to the UK.
declaration of war restrictions
We don't have any.
sending of expeditionary forces to AI alliance partner to save on supply drain
Once again, an unnamed player used this in total disregard of the spirit of the game. We banned it.
tech sharing
(after MUCH adjusting/tweaking/play-testing)
Every country may share tech once per month to any country they choose. The nations of Germany, UK, USA, USSR, and Japan may only receive "silver" techs. France, Italy, Nationalist China, and everyone else may receive "gold" or "silver" techs. No doctrines (land/sea/air) may be shared ever. AI tech sharing (obviously) has no bearing on these rules.
supply convoys
"neutral" supply of beachheads during conflict is forbidden.
alliance switching
We generally permit alliance switching (for example France joining the Commintern) although this has been under heavy debate recently. While it hasn't happened in our games, the USA "exploitation" into joining the Axis or Commintern does lead to a need for some kind of restriction
commonwealth force control
After a player who shall go unnamed, joined the Allies and immediately took control of the Canadian, South African, and Australian forces to engage in a Greek campaign against a neutral Turkey - military control of Commonwealth partners should be restricted to the UK.
declaration of war restrictions
We don't have any.
sending of expeditionary forces to AI alliance partner to save on supply drain
Once again, an unnamed player used this in total disregard of the spirit of the game. We banned it.