This is actually a very interesting thread. Im pretty new to the business and havent had any full service accounts (bed and flower maintenance year round)
If i were to guess, Id start by laying down preen in the spring? along with every few months. Check the leaves, top and underside, for any disease/pests every week. Stay on top of pulling weeds every week. Only put down pesticide when needed. Make sure you stay on top during times of drought, stressed plants are more susceptable to disease and pests in most cases. Not sure on how much horticulture background you have but Id identify some of the main species and learn some common problems they have.
Im interested in hearing what people have to say
EDIT- I agree with smallaxe, no way id sub this out. This is one of the most fun and rewarding things in horticulture, proper cultural maint, diagnoses and control.