I know there are some rules you follow after experience with toys. Some are based on experience, others common sense, others just your own preference and personality. I'll start with a few, and you can feel free to add your own.
THOU SHALT NOT SKIP WARMING UP!- This is especially important with larger toys. You're dealing with sensitive areas that only relax when you allow them to, and warming up will help to avoid pain and suffering. A little lube, a finger, a smaller toy... these things are priceless.
THOU SHALT KEEP A DRY HAND!- Okay, so this one is from personal experience. As technology advances, you end up with vibrators with different types of dials and controls, and materials that are more slippery than a dog in a bathtub. If you allow both hands to become lubricated, trouble will follow. Try turning up or down a rotating base of a vibrator with a slippery hand. Frustrating! Follow the rule- one hand for lubrication, one hand for control.
Feel free to add your own!