8. Positioning
This isn't just about keeping the boss or mob facing away from the party/healer.

The last boss in copperbell: If you keep the boss parked near the last wall the adds run to break down, melee DPS does not have to run back and forth between the boss and the adds everytime an add appears.

1st boss in Stone Vigil: If you keep the boss in the middle of the arena, when he breaks away and runs to a wall in order to AOE breath, everyone has more time to get out of the way, because he takes longer to run to the wall.

2nd boss in Stone Vigil: If you keep the boss near the entrance the cyclones have further to travel and again PT members have more warning.

Zfz's example in Brayflox above: If you park the boss on top of where the adds spawn then you can easily flash them the very moment they appear.

Watch for what a boss does movement wise and where adds or mechanics appear from. Moving the boss to an appropriate spot may completely change the fight for the better.