Given that the question has restricted me to “the most important maintenance tip” for pier and beam foundations, I would have to say, assure that drainage is properly maintained. The same is true for concrete slab foundations.
Absolutely no water should be allowed to accumulate near a house, run toward the foundation, or pass under the house.
With our clay soils, water causes the surface and sub-surface piers to heave and sink or drop with seasonal changes in moisture. A damp, enclosed pier and beam foundation area may also lead to wood rot and encourage termite infestations. Assure that ventilation is adequate, and if not, add vents and fans.
More often than not, most of the damage to a pier and beam foundation will come from inadequate pier support as the wet clay turns to mud.
