1. Don't prune before January 1
Try to finish pruning before bud-break (although until bloom is allowed); don't prune when it is very warm or predicted to get very cold.
2. It's all about farming the sunlight
Apple and pear wood needs to receive adequate sunlight (50% is commonly cited, whatever that means?) to form quality fruit buds and produce top-quality fruit. Just remember, shade is your enemy. Although not easy to accomplish, fruit buds in the bottom of the tree should get nearly as much light as buds in the top of the tree. (This is why very narrow canopies, as in a fruiting wall, are in vogue.)
3. Leave the hand-pruners in the shed
Larger apple and pear wood (4-years old and older) needs to be removed during dormant pruning using a saw and/or high-quality lopper. (I like Hickock or these -- and only these -- Corona loppers.) A small chain saw and pole chain saw should be in every pruners arsenal when dealing with semi-dwarf trees. (And sometimes dwarfs that have gotten out-of-hand!)
4. Avoid heading cuts like the plague
Use thinning cuts to simplify lateral and/or scaffold branches; more simple lateral and/or scaffold branches are preferable to fewer complex branches; thinning cuts typically allow sunlight to better penetrate the apple or pear tree canopy.
5. Limit tree height to no more than 90% of row spacing
If row spacing is 12 feet tree height should be limited to 10 feet. (Yea I know my math here is suspect, but round to the nearest foot less than 90% after doing the calculation -- who can relate to 10.8 feet?, and 11 feet is too tall!) "Crop-and-flop" the central-leader, i.e., don't cut the leader if at all possible to limit tree height until it has "cropped and flopped," then cut to a weaker side-shoot to limit height.
6. Use diameter-based pruning, particularly on the central-leader
If a branch is more than 1/3 to 1/2 the diameter of the branch (or central-leader) from which it originates, cut it out using a thinning cut; I've been leaning to using the 1/3 diameter cut-off more and more...
7. Forks belong on your dinner table, not in your orchard!
If you can picture it, it is corollary to 6. above.
8. Use the 1-2-3 rule, particularly in dwarf, spindle-type orchards
After pruning, you should have a balance of 1-year-old, 2-year-old, and 3-year-old wood; 2- and 3-year-old wood is fruiting, 1-year-old wood is left to become 2- and eventually 3-year-old wood; 1-year-old wood is preferably pencil-diameter and 12-18 inches long and left UN-HEADED!!!
9. Cut-back (or remove) wood that droops below 45 degrees from horizontal
Pendant wood is weak and will produce smaller, poor-quality fruit. I know, I know, this would arguably be a heading cut, but if you are ever going to use a heading cut, by all means, use it into weak wood! Of course, also cut out (using a thinning cut!) vigorous, upright shoots and water-sprouts.
10. Remove one to several of the larger branches
Except on central-leader trees where a permanent lower-scaffold branch structure has been developed, in general it is desirable remove the 1, or 2 (or 3) biggest branches (using diameter-based pruning as described above) originating on the leader, particularly in the top of the tree.
11. Favor a palmette-shape tree top going down the row
If your rows are planted in a North-South orientation (as they should be!), favor keeping branches in the row vs. into the row -- this helps prevent shading and creates a palmette-shape top of the tree; I use this when making decisions (based on diameter-based pruning) on which bigger (older) branches are being removed (using a thinning cut!) in the top of the tree
12. Experienced pruners, read this
13. Beginning pruners and homeowner, read this
Did I miss something? Feel free to leave a comment...