Watch your wires! If any of your lights have frayed cords or loose sockets, throw them away and get a new one. It won't cost near as much as fixing damage caused by an electrical fire.
Make sure your friends unplug all their decorative lights before leaving home or going to bed. If they prefer to have their lights on until they've fallen asleep, suggest that they use a timer that turns them off shortly after they've fallen asleep.
Make sure you keep your friends safe from shock hazards with their decorations: If you notice overloaded electrical outlets, extensions cords that are run under carpets, across doorways, or near heaters, say something! Also, if they have a metal Christmas tree, ensure that they don't decorate it with electrical lights.
If your friends use a real tree, it should be kept in a large, deep, non-tip stand, well away from exits, fireplaces, heaters, and any other heat sources. When they dispose of it, it should be kept outside, a safe distance from their house.
