Always inspect lights before you hang and plug them in. Check for frayed wires, broken bulbs and sockets, and loose connections—those problems indicate fire and electrocution hazards. Keep in mind that the larger a bulb is, the more heat it'll drive, so inspect surfaces after your first light up to monitor for possible damage to gutters. Lead is present in many Christmas light strings; do your homework and read packages carefully to determine the right set of lights for your home.
Regardless of your preferences (colored, uncolored, large bulbs, or small) make sure the box you pick up has a UL seal on it. UL-certified products have undergone testing for durability and safety. Finally, use as few power cords as possible to prevent overloading and only use lights marked for outdoor use in your exterior lighting displays. When you're not enjoying your beautiful sparklers, be sure to unplug.