How to Prune Trees
When pruning, it is important to remove wood all the way back to another branch, or bud. You should prune first for health, removing any damaged or dead wood, and then for architecture. Look at the tree, and picture it 10 years down the road. Look here for more information: http://www.na.fs.fed.us/spfo/pubs/howtos/ht_prune/prun001.