Buy a copy of The Fundamentals of General Tree Work by GF Beranek. Read it, even the parts that pertain to work you won't be doing. It will give you a good sense of how trees operate, the forces at work, and how to manage/predict/control/avoid them.
Otherwise, it's a matter of experience. I've found that you can teach someone a LOT about how trees work by setting them up limbing a downed tree with a slow saw (the reed valve Echos or old Homelite XL/192/200 saws are great for this). The slow saw allows you to see how the wood is reacting, to see how tension and twisting forces operate, and to become better at predicting what a standing live tree will do.
Also consider watching some of EKKA's videos, which may still be linked off ArboristSite; they can also be found on his site and on youtube. You can learn a thing or two on there, for sure. There are also some other good videos (primarily northern European/Scandanavian in origin) on youtube that you may wish to look at. Poke around, you'll soon have an eye for when people are doing things right and when they're doing them wrong.
Once you're out in the field, keep your gear running right. Don't over-bar your saws, don't run dull chains, don't skip your safety gear, buy and use your wedges (buy many!) for felling and bucking, and don't hesitate to put a proper line up in a tree if directional precision is required. Do note, however, that if you are going to put a line in a tree, please have the sense to use proper rope, not that crap from Home Depot. I used the HD stuff for years in my teens, and while I'm still here to tell about it, I feel much better tensioning a limb or whole tree with a length of 3/4" StableBraid tied off to my Jeep than I ever did with my 1/2" braided non-name unrated poly crap from HD. You can get great deals on odd-length pieces of professional-grade name-brand ropes on eBay. A couple 60-80' lengths of Stable Braid, a throwbag and line, and you will be MUCH better off than 99% of home woodcutters when it comes time to put tricky trees down.
Beyond that, study what you're doing. You'll soon figure out where your mistakes are and what parts of the process are intuitive to you. Learn from mistakes, aim for perfection in cutting and safety, and don't get overconfident - I've never hurt myself dropping a tree that I respected, but I have put a saw into my thigh a time or two (in the days before I wore chaps) when trimming out "easy" scrubby 3" trees.
