What maintenance tasks and intervals protect two diesel pickups from failures?

[AaronZM]

I run a small landscaping business with two older diesel pickup trucks, and I'm trying to get ahead of major repairs by implementing a more rigorous preventative maintenance schedule. I'm familiar with the basics like oil changes and filter replacements, but I'm unsure about the optimal intervals for more costly items like fuel injector cleaning or transmission fluid flushes on high-mileage work trucks. For other owners of diesel fleet vehicles, what specific maintenance tasks have you found to be most critical for longevity, and are there any warning signs I should watch for that typically precede expensive failures?