Decorative lighting is a necessary part of each interior design and as an electrical appliance needs maintenance from time to time. We are used to set it and forget it - after the initial install we do not pay any attention to it, unless there is a problem - for example to change the burnt bulb. Unfortunately, there is much more to be done to ensure that you will get the most of your lighting fixtures and feel comfortable in your home.
Cleaning - over the time the protective glass, lampshade or any other kind of diffuser gets dirty. A lot of dust and other particles stick to the lamps and decrease the illuminated light by 30% to 50%. You should consider the fact that different rooms are delivering different volume and type of dirt. Kitchen pendants tend to collect dirt faster than lighting fixtures in the bedroom. That's normal as there are a lot more vapors in the food preparing rooms. Consider taking off the glasses, diffusers and lampshades regularly to clean them.
Replacing bulbs - all bulbs are aging and suffer from reduction of light emitted. Energy saving bulbs or also known as Compact Fluorescent Lamps are losing 20% of their luminous flux every year. If you combine that with some dust, then don't be surprised if your rooms are too dark. It is a good idea to replace old bulbs every two years.
Power supply revision - modern light bulbs like LED or low voltage halogens are supplied by a LED driver or transformer. Even though such devices can significantly increase the life of the bulbs, they should be checked on a yearly basis in order to provide the best electrical parameters to the bulbs. Because these devices are generating a lot of heat, it is required to not allow the dust to cover them and cause overheating. There is also a problem called "loose wire problem" - when the wires are not tightly secured to the power supply they can start burn the isolation and damage the whole lighting fixture.
Flexible arm lighting - there are some lamps and pendants that allows you to change their form or angle of the light being emitted. Using them often can loosen the flexible parts and ruin them, so whenever you need to adjust their position, make it smoothly and gently.
Waterproof lights - you should inspect their covers and diffusers on regular bases to assure that there is no any chance of water getting inside and make a short circuit. Such lighting fixtures are installed in bathrooms so do not neglect their integrity, if any doubt - replace them immediately.