It is easiest to start with young fruit trees, because they develop best when you “train” them through pruning. In terms of apple trees, you can pick the strongest “leading” upright trunk to structure the rest of the growth (scaffold branches) around in equal measure. By focusing on the strongest branches and cutting away the competing growth, weak or diseased limbs, and sucker branches, the tree will direct its energy in an efficient way (and you don’t risk the breaking of weak branches when the weight of the fruit shows--scaffold branches are strongest at a 90 degree angle from the trunk).
With all fruit trees, you need to do pruning and maintenance each year. With mature trees, you would probably need to do one heavy pruning the first year followed by some lighter pruning and maintenance in the following years in order to see some good results. You might think that cutting so much off the tree would harm or kill it, but if you use sharp tools, disinfectant, and you know where to make the cuts (not through the collar of the growth, but still close to the union), you will actually improve the tree’s growth.
Fun fact: ever wondered why they call it a manicured garden? Manicure comes from the Latin for hand + care, “to care for by hand”. Pruning trees is a good example of a healthy relationship between man and trees--you can improve the yield of a fruit tree by using proper techniques, without hurting the tree itself. This is organic gardening at its best.
You can watch this video in order to get a better idea of the shape of different training regimes, and exactly how to make the cuts:
http://www.youtube.com/watch?feature=player_embedded&v=LxZvNR_N3kQ
Something I don’t usually do, but is an option for trees plagued by certain insects and aphids, is applying a dormant spray: it is an oil which kills these pest by suffocating them -- organic products are available. Here’s another video.
http://www.youtube.com/watch?feature=player_embedded&v=KAgXhFGagM0#!