Know the territories that you absolutely cannot let anyone else control:
Austria-Galicia, Serbia, Tyrolia
England-North Sea, English Channel
France-Burgundy, English Channel
Germany-Ruhr, Baltic Sea, Heligoland Bight
Italy-Piedmont, Tyrolia, Ionian Sea, Tyrrhenian Sea
Russia-Black Sea, Galicia, Ukraine, Livonia, Barents Sea
Turkey-Black Sea, Aegean Sea, Eastern Med, Bulgaria
Now granted, occasionally, another power will wind up moving successfully into a must-have territory, but you have to make sure they don't have the means to stay there.