Keep it clear
Leaves, pine needles and palm fronds sitting on a roof hold moisture from dew and rain against the surface, and in the valleys they dam water so it backs up under tiles and shingles. Clear them off after windy spells, and clear the gutters at the same time so what comes off the roof has somewhere to go.
On tile roofs, debris in the channels between tiles is especially bad because you cannot see it from the ground.
Trim the trees
Branches that touch or overhang the roof do three things: they drop debris, they scrape the surface in wind, and they give squirrels and rats a bridge to your eaves. Cut them back so nothing overhangs, and you will notice the gutters stay cleaner and the roof dries faster after rain.
In the foothill neighborhoods this is also part of your ember defense.
Let the attic breathe
A hot attic bakes shingles from underneath and ages underlayment under tile. Make sure eave vents are not blocked by insulation and that there is an exit at the ridge or gable for the hot air. If your attic hits well over a hundred degrees on a summer afternoon, that heat is shortening your roof's life.
Adding ridge ventilation during a re-roof is one of the cheapest life-extenders there is.
Check the flashings each fall
Pipe boots, chimney counterflashing, skylight curbs and wall junctions are where roofs fail. Once a year, before the rains, someone should look at each one and reseal or replace what has split. It takes an hour and prevents most of the leaks I get called to in January.
If you are not comfortable on the roof, this is exactly what my yearly plan covers.
Act on small signs
A single slipped tile, a shingle in the yard, a faint stain on the garage ceiling. Each of those is a cheap fix on the day you notice it and an expensive one a year later. The roofs that last longest in this valley are not the ones with the best materials; they are the ones whose owners respond to small things quickly.
Write down what you notice and when, and mention it at the next inspection.