One variant that works well with Wilson Diplomacy (where all discussions are at the table and in the open in five minute rounds) is to have what I call Formal Diplomacy.

Each player has in front of him two areas to hold other player’s pieces or equivalent chits, or you can just write it down on a piece of paper: one area for countries they are at war with and the other for countries that they are allied with. All other countries they are neutral towards.

The formal relationship between the powers dictates what you may or may not do to each other under each of the various states of hostility. It also is a nice way to do away with such awkward problems as the unwanted convoy, screwing up an intended self bounce by supporting what amounts to your enemy’s piece against himself, as well as some other awkwardness of those who like to see things a little closer to the historic norms. This still allows for sudden stabs and declarations of war, but it does give a more Formalistic Patter to the game.

Restrictions are as follows:

For Allied Powers

• Move: You can move through their home provinces and their owned provinces. You cannot dislodge their units under any circumstances.
• Support: You can receive and give support for each other’s pieces. If you move on a province that is occupied by them and that issued support, however, you do not cut support, just as you cannot dislodge them.
• Convoy: You can convoy each other’s armies.
• Centers: Ifyou occupy your allies’ center you do NOT take control of it from them.

For at Neutral Powers

• Move: You cannot move to a province that contains their piece. You can order there in the hope that they are successful in moving out, but your movement there is void and treated as a hold if the neutral does not move. You cannot enter a neutral power’s home country or any of his owned supply centers. You can both move to an un-owned supply center, province (North Africa and Albania are the only un-owned non-supply centers) or sea province and bounce each other there but not dislodge the other. Example: Germany
  is neutral towards France. France orders Burgundy to Belgium (un-owned) and Germany moves Ruhr to Belgium supported by Holland. Then Germany gets in since no one was dislodged and Belgium was un-owned to begin with.
• Support: You cannot cut the support of a neutral piece since you cannot move into their occupied space. You cannot issue support for a neutral power’s piece.
• Convoy: You cannot convoy a neutral power’s army.
• Ownership: You cannot enter, let alone take, a neutral power’s supply center.

For Powers at War

• Move: You can move on any of their pieces anywhere even if that would mean moving on a neutral power’s home area if it is occupied by a piece of the designated enemy. This way a neutral cannot set up a sanctuary for your enemy.
• Support: You cannot support a piece of a power that you are at war with.
• Convoy: You cannot convoy an enemy’s army

Declarations:
At the end of the 5 minutes of negotiations BEFORE the writing of orders, all Declarations are written down in secret and then exposed. Declaration of Alliance must be made in conjunction with the other power for them to be effective. Declarations of War are unilateral. A state of Neutrality is the automatic default for countries neither at war nor allied to each other. During any Declaration period, an Alliance member may break the alliance unilaterally. It takes mutual consent to end a War. Declaring Neutrality or Alliance may do this. Even if these declarations are mismatched the default then applies and the countries are neutral towards each other.

Neutrality violations:
Any unit that is in violation of neutrality at the end of the movement phase is interned by the neutral power and disbanded.

Home Non-Supply Center provinces:
All the powers have non-supply center Home owned provinces:
England: Clyde, Yorkshire and Wales
France: Burgundy, Picardy, Gascony
Germany: Ruhr, Prussia and Silesia
Austria: Tyrolia, Bohemia and Galicia
Italy: Apulia, Piedmont and Tuscany,
Russia: Finland, Livonia and the Ukraine
Turkey: Syria and Armenia

These provinces are considered owned by the original power unless they do not have an equal or greater number of owned home supply centers adjacent to it compared with enemy occupied home centers. Example: Russia occupies Berlin. Prussia is now considered part of Russia while Silesia is considered part of Germany as long as Germany owns Munich.

Albania and North Africa are always considered neutral provinces when not occupied.