Well the first thing to realize when it comes to armour is that it will always be a tradeoff between comfort/mobility and protection. You can rarely have max levels of both and have to opt for one or the other.

The number one thing to keep armour comfortable for the long term is to wear as little as you can get away with under the armour. In the age of full plate armour, this being the 15th century and onwards, regular under armour wear would be a doublet with little to no padding on it. People without full armour also regularly just wore their civilian doublets or coats under it.

One of the worst offenders when it comes to becoming too hot in armour is face protection. A helmet which covers the face does impact O2 circulation. Adding breathes helps to a degree but add too many and you compromise the structural integrity of the helmet. A good compromise was reached when hinged visors were invented, which allowed a visor to be more protective (and at the same time restrictive) with the knowledge that you can raise it when needed. One person which talks about this is Pietro Monte. In Collactaena, he says this

If someone knows how to parry and the enemy is strongly armed, it is often useful to have a visor which goes upwards, because our breathing endures more and it is also easier to see what must be done, and almost no danger threatens, or will rarely happen, especially if we have iron gauntlets to turn away the other’s weapon.

If one looks at period depictions of infantrymen especially, these rarely have any face protection on their helmets. Part of it is likely due to cost, as the Howard Accounts mention visored sallets costing over three times the amount of unvisored ones, however even infantry with a lot of armour seem to skip out on visors. Part of the reason is likely because of long-term comfort. Even if you can raise your visor it can be annoying and fall down again. Infantry spent a lot more time in their armour than knights and men-at-arms did, so it would make sense for them to try to maximize comfort where they can. As Pietro Monte also says above, the face is among the easiest parts of the body to protect and you can afford to not armour it as much.

Other examples apart from helmets comes when travelling. Brigandines (a type of armour made from many smaller plates riveted to a textile covering) were seen as more comfortable than full plate cuirasses and are with decent regularity used when travelling over those. In fact, in the Burgundian Ordonnances, the knights are explicitly told not to wear brigandines. It says something along the lines of 'not be armed in the manner of coustilliers' when travelling (a coustillier was a lighter horseman, which in the ordinance are told to wear brigandines). Knowing the burgundians this was probably to do with status, but it indicates that the temptation to wear a brigandine would be high. They are told which items they are allowed to take off for comfort. For example the pauldrons and bevors.

I do realize that this is pretty heavily late medieval, because that is what I focus on most. The first point however, of wearing as little as possible under the armour, holds true for most of history. Unlike modern re-enactors who wear a lot of padding because they have to go for safety and short-time comfort when getting hit, most historical people were faced with different problems and situations and thus acted differently.