Deceleration lights blink when a car slows down, giving the driver behind that car extra warning. The idea is to reduce rear-end collisions; the blinking lights are supposed to grab a driver's attention and give them a chance to slow down. Once you install the lights, there's not a lot you have to do other than occasionally checking to see if the lights are still working. What little you do have to do, though, is vital for using these lights correctly.
Keep Your Window and the Light Clean
Deceleration warning lights tend to be an amber color. Keep the light and the window it's behind as clean as possible to help with visibility. If you have a very dirty window, that could reduce the ability of the following driver to see the lights blinking, especially from a distance during the day. There's not much you can do about bright sunlight obscuring the view of the lights, but you can at least prevent dirt from getting in the way.
Do Not Increase the Blink Rate
Deceleration lights are supposed to blink at a specific rate. For example, in California, single lights that are centered on a vehicle have to blink in proportion to the rate of deceleration; dual lamps on the sides are restricted to four flashes in four seconds. Do not tinker with the lights because you want a faster blink rate. Not only does it violate the law, but if you hit the wrong rate between 5 and 30 blinks per second, you could trigger a seizure in a passenger in the following car if the passenger has epilepsy. While there aren't studies linking deceleration light blink rate to this, a specific range of light-flash rates is known to cause a problem. Don't take a chance.
Place the Light Per State Law
Speaking of state law, these lights have to be placed properly. This is not the time to get fancy and try to form patterns with the lights. Stick to the number of lights allowed and the placement determined by your state law. If you plan to drive into another state, double-check the laws there to ensure you won't be violating any of them.
Vehicle deceleration warning lamps are very helpful and do prevent at least some collisions. They're an excellent addition to your car and are easy to take care of. With the right placement and care, the lights should last a while.