A gun is ALWAYS loaded, so treat it accordingly.
Keep your finger off the trigger until you're ready to shoot.
Never point your gun at anything you're not willing to destroy.
Be aware of your target and backstop.
Assumptions are dangerous. I assumed the gun was unloaded and did not double verify that status as rule one would imply. We can get away with a lot of assumptions that don't risk lives. We can assume the oil level is correct in our car. Of course, if that assumption proves false, it's time to fork out the cash for a new engine, but we'll live through the experience. When we make assumptions about the loaded condition of a gun, however, we better be right. At minimum, the consequences of a false assumption are embarrassment or property damage. In the worst case, it can lead to loss of life.
It's critically important to follow an exact and specific procedure before handling a gun. Do the exact same steps every single time. Verify that the gun is unloaded. I like to put the ammo from the gun near my dry-fire target, so as I am pulling the trigger, I can visibly verify that the ammo for the gun is not in it. The specific procedure is important. That's why 20,000-hour veteran airline pilots still use written checklists for things they've done thousands of times. The consequences of forgetting even a single action can be catastrophic.
In lives full of distractions, it's very easy to lose focus for a split second--even when handling guns. Who among us can say they've never left a door unlocked? Or missed a stop sign? Or made any other mistake from a momentary lapse?
Shooting your dining table does not increase the level of marital bliss.