Be Realistic and Be Professional!
It is important to have goals and solutions that are obtainable. Keep in mind that other countries have their own needs to worry about.
Be careful about what you say (and what you believe).
Plan your communiqués carefully and consider whether or not other parties are telling you the full truth. Diplomacy is more perception than reality.
Seek common ground.
Think about what compromises you are willing to make that may be attractive to the other countries. Be flexible but stay in ideological character.
Be patient. If at an impasse, try again later. Or try another nation. Leave diplomatic “avenues of retreat” open.