1. Always 'look' with your ears.
2. Never extend downwind. I you do, that will cause heaps of trouble for others.
3. Extend upwind if necessary, but do tell ATC.
4. If someone is doing mega circuits, note their call sign and speak on the ground.
5. Go around at circuit height is the one ahead is flying an enormous circuit. Tell ATC.
6. Personally, I will cut in very tight, but not if the one ahead is on finals. He has priority.