Cass Turnbull's PlantAmnesty.org has so much great information on pruning trees, it's hard to summarize it in a few paragraphs. Most of it is in a regular column called 'Pruning Tips'. But there is also the Hardy Plantsman, Plant Health Care and other Miscellaneous articles. Here are a few highlights:
http://www.plantamnesty.org/6024_pages/index.shtml