a) Replace like with like (don't change materials or construction techniques)
b) Make any repair work obvious (don't try to pass new fabric off as original)
c) Repairs should be reversible.
It's a complicated field, but the general spirit is that when a building needs to be repaired, you try to make the new elements match as closely as possible to the way the structure was when it was first built. You don't say "we know better than the medieval masons, so we'll use reinforced concrete instead of stone" because that wouldn't be a restoration. It would turn the building into a hybrid of old and new technology, when what everyone wants is to experience the old way of building.
There's also an element of arrogance in trying to improve on medieval architecture—most people would agree that architecture, as an art, isn't something that progresses inexorably like science. The cathedrals are already as good as architecture gets. "Updating" a medieval structure to use the trendy structural technologies of the 21st century is a barbaric idea, because it is literally a case of destroying or interfering with an irreplaceable artefact.
Obviously, not every old structure has to be treated with kid gloves as a priceless artefact. But I hope you can see what I'm getting at when I say that "best practice" in building conservation means not acting as if you know better than the original builders. It's a case of respecting the original design and not messing with it for the sake of employing modern technology.