Having healthy trees is primarily a frame of mind. Of course, what you do or do not do to your trees is very important, BUT having a tree-healthy mind set allows you to make the right choices and decisions.
The most important thing you do is visually inspect your trees seriously all the time; they are enjoyable to look at, aren't they? By constantly watching your trees and getting to know them on a more intimate level, it will become impossible for you to miss a problem that needs addressing.
Trees are not static, they are alive, dynamic and so is the world around them. Everything is constantly changing, tree growth, insect movements, disease invasion. By regularly inspecting your trees you will catch the signs of drought, the first signs of a insect attack or the early signs of the presence of disease.
You need to SEE, not just look. Don't skip over the tree; move your eye slowly up and down, back and forth. Once you get used to doing this and get to know your tree on a deeper level you will not miss any abnormalities.
This is the best piece of knowledge I can give you -- learn what is normal and healthy, observe the tree regularly, noting any detail that is irregular. Once noticed, it can then be addressed. Maybe the tree just needs more water, perhaps a treatment to deal with an aggressive insect, or pruning to remove the first signs of disease.
This is exactly what I mean by bringing love to the tree, taking the time to pay attention. No one else can do this as well as you can -- you live there. Paying people to care for your trees is ineffective; most will not take the time required, do not have the eye or are too rushed to do it. Commercial businesses want to keep selling you product, fertilizers and insecticides that you don't need and that are not good for the world. You don't need these treatments because through observation and proper watering practices you have already kept your trees healthy.
When the natural healthy state of the tree is a pattern in your thinking, any abnormality will jump out at you when you do your regular inspection.