There are some good points made by others in this forum, and some wrong ones too. I do this stuff for a living, I'm a master arborist, so I know a thing or two about pruning trees.
Figure out what kind of tree you're dealing with first, this will determine your strategy. Some trees are better pruned in winter, some in summer, some can take a hard prune, some will react badly to being pruned at all.
Figure out how old and vital your tree is, you can tell by how much growth it put on the previous year by looking up into the crown to see how much the end twigs grew. Good growth means good healing potential, not so much means not very good healing potential. How vital it is determines how much you can prune without damaging the tree.
First rule of thumb: Trees HATE being pruned! so with that in mind, every pruning action should be viewed as a minimization of damage and stress for the tree, while accomplishing whatever goals you have for it - it's a compromise. One person said that most people cut too little - NOT true, most cut way too much.
Second rule of thumb: Crown reductions don't work (i.e. making your tree smaller), at least not for long. I always try to advise my customers away from this action. The tree can't provide enough food for itself so it has to dip into its savings account to survive and bud out the following year.. This is hugely stressful, which sends out a signal for all wood destroying insects in the area that the buffet is open. Plus, the tree tries as fast as possible to replace the lost leaf mass, and does so by sending up loads of water sprouts. These sprouts then grow rapidly to the same height the tree was before, but are all in competition with each other, poorly connected with the main branch, provide more shade then before, and are more prone to breaking out, all while making the tree look totally shitty. Meanwhile, the wood destroying fungi are at work munching away at all of the new pruning cuts.... Don't do it!
Third rule of thumb: 1/3 of the leaf mass may be removed from the tree or any given branch, NOT 1/3 of the height. Remember, leaves are how the tree makes food, and a loss of 1/3 of them is about as much as a tree can take without reacting too badly. That means thumb-thick branches can be drop-crotched for a reduction.
Fourth rule of thumb: Crown maintenance - dead wood first (cut, don't tear it out), then diseased or dying branches, then crossing branches and water sprouts. That's it! This action makes a lot of sense, and can correct failures before they become an expensive issue in the future.
Fifth rule of thumb: 'Thinning out' is harmful. Other than the above maintenance, the crown should not be opened up, it allows sunlight and wind to get in, causing sunscald, water sprouts, and breakage of branches.
Sixth rule of thumb: There are two groups of trees categorised according to their resistance to wood destroying fungi. Trees with weak defences should never have wounds greater than 5 cm (Birch, Horsechestnut, Poplar, Willow), stronger trees can be pruned up to 10 cm (oak, hard maples, conifers, beech). If you need to cut a branch bigger than this, there should be a good reason for it, because it will cost the tree in the long run.
Seventh rule of thumb: Proper pruning cuts have been well described in this thread, first take the weight off, then make a finishing cut at the branch collar. When reducing a branch, find an appropriate side branch no less than 1/3 the diameter of the main one, growing in about the same direction, to form the new branch tip.
Eighth rule of thumb: Leave enough terminal twigs on the tree. Terminal buds contain a hormone that suppresses side growth, and keeps dormant buds dormant. Not only does it preserve the natural beauty and form of the tree, but it reduces the amount of water sprouts that will grow back.
That's enough for now, but there's way more to it, I'd be happy to answer any specific questions that are out there!