I don't have too many rules, but here goes the "my practices" list:
Always wash and press fabrics before cutting (except jelly rolls, layer cakes). I serge the raw edges before washing.
No sewing over pins
Always unplug any iron and shut off any sewing machine if I leave the room, even for a minute (because you never know if a distraction will occur while you're away)
All machines plugged into a surge protector (and when I get my longarm, a pure sine UPS)
Only one project on the cutting table at a time (so I don't accidentally mix up fabrics).
Measure twice, cut once. When unsure, measure again before cutting.
New needles for each quilt-sized project, both for piecing and for quilting.
Clean out the bobbin area and oil the wick before starting a new piecing project or starting the first quilting on a project.
Fabric scissors stored in a cutting table drawer when I'm not in the room (so someone doesn't use them for paper).
If I make a mistake and have been working for a while, it's time to stop for the day.