One approach was to maintain a defensive fight that was economical of energy, as when a Portuguese named Diego d’Ollumen fought against the Breton Guillaume de la Haye in Paris in 1415.
And the Portuguese came very boldly and joyously, seeking to strike his adversary. But he always put aside his blows, without doing anything else. The fight continued for some time, but he still remained on the defensive as he had been advised. Often the Portuguese lifted his visor, and made signs to the other that he should do likewise. When the fight had continued for some time in this way the Portuguese lifted his visor and Guillaume de la Haye, without lifting his, sought to present the point of his axe to his face. The Portuguese began at once to retreat, but when they saw how it was going they cried “Ho, ho, ho” and went diligently to take them. They say that the Portuguese was very short of breath, and that if de la Haye had wanted to come a little closer he could have thrown him to earth in wrestling, as he was one of the best wrestlers you could find. Then both of them were given honor and good cheer.
Jean Juvenal des Ursins Histoire de Charles VI, Roy de France in Nouvelle Collection des Memoires pour Servir a l’Histoire de France Paris 1836 Vol. 2 p. 503
Another tactic was to fight so that your helmet let you breath more easily than your opponent, either without a visor or with your visor raised. Chastellain's account of the Lalaing vs. Douglas fight in Scotland in 1449 reported that Jacques de Lalaing fought without a visor and could breath freely and his opponent, James Douglas, who fought with a closed visor, could not. He added that Lalaing wrestled against Douglas in order to put him out of breath.
Sir James fought in his basinet with a closed visor, and Jacques was without a visor so that he could breath freely, and that of Sir James was quite the opposite. This was easily seen when king threw down his baton and and the visor was raised.
Georges Chastellain, Chronique de J. de Lalain ed. J. A. Buchon (Paris, 1825) p. 203
You could improve your chances of prevailing by being in better physical condition than your opponent. Boucicaut was known for his rigorous physical training.
And now he began to test himself by mounting a courser in full armor. At other times he would run or hike for a long way on foot, to train himself not to get out of breath and to endure long efforts. At other times he would strike with an axe or hammer for a long to be able to hold out well in armor, and so his arms and hands would endure striking for a long time, and train himself to nimbly lift his arms. By these means he trained himself so well that at that time you couldn't find another gentleman in equal physical condition. He would do a somersault armed in all his armor, and dance armed in a mail shirt.
Froissart, Jean, Jean Alexandre C. Buchon, and Jean Froissart. Vol. 3 1812. Les chroniques de Sire Jean Froissart... / [Et du] Livre des faits du bon Messire Jean le Maingre, dit Bouciquaut. Paris: Soc. du Panthéon litt.
Dom Duarte of Portugal, writing in his 1434 Regimento para aprender algunas causas das armas also advised endurance conditioning.
On the third hour (Tierce) on some days he goes to practice. And to practice he should arm himself with all weapons and goes on foot a great distance up a (hill) for a long length (piece or section) to strengthen himself.
Translation by Steve Hick in Oakeshott, R. Ewart. 2002. SPADA: an anthology of swordsmanship in memory of Ewart Oakeshott. Union City, Calif: Chivalry Bookshelf.