Pruning techniques are specific to the plant material being pruned, making it hard to identify any “universal truths” to draw upon. However, there is one such “truth” that comes to mind when discussing aesthetic pruning for specimen trees. It is this: consider that, first of all, you’re “growing trunk.” Thus, emphasis is on what I consider a “bottom-up” approach to tree design.
Trees take many years to mature. These are years of your dedication and your resource investment. In this lengthy development, concerning oneself too early with canopy is a distraction from your main concern of growing a great trunk. Consider these ideas when you prune.
Identify the purpose for your tree. - This includes where it will be located (foreground or background), how big you see it getting for the location (scale it to human proportion), and how central is it to the overall garden design (is this a supporting or main feature). Work to clarify that purpose.
Design your tree from the bottom up. - When making decisions about what and where to make cuts, start at the ground level and work your way up the tree and out to branch tips. This means initially determining the primary “leaning” of the trunk (is it wind-blown, upright, etc.). Then, season-by-season, year-by-year, look closely at the secondary branches that emerge from the trunk. Over time, you will selectively reduce them, simplifying the structure. Only then will you finally be attentive to the tertiary branches that support the canopy: keep it open and airy. To be clear, these are not one-time pruning cuts. Instead, they are a strategy that you employ, repeat, and modify.
These concepts hold true for most evergreen trees (pines as well as broadleaf varieties) and for deciduous trees (such as Japanese maples). Choose which branches to remove by looking for errant ones: misshapen, dead or dying, becoming too large, growing in a contrary direction, or crossing over or crowding other limbs.
I recently spring pruned the nine camellias in the tea garden along the Dewy Path with this approach. They are generally about 10 years old and it was finally time to be bold with their shaping. They all came out beautifully and look and feel much more mature. This moves them closer to their planned role and purpose as broadleaf canopy over the dewy path. This is an ongoing task that brings its own annual rewards and, if lucky, an occasional “ah-ha!”