I have a lower back pain due to working long hours at my desk. What should I do?

Low back pain. The best treatment are things you can do away from your desk. Your core muscles are weak from sitting posture. Walking, crunches, pool time.. You get the idea . I don't recommend belts or straps they just take away even more muscle tone. The only item i would recommend at your desk would be standing, stretching and trading chair for balance ball. Trade off/on for chair when starting.
Lumbar support. Back pain at the desk is usually the result of poor sitting posture. Using a good lumbar support will cause your pelvis to shift forward reducing the stress on the lumbar discs. You will also want to get up and walk aroung every 30 minutes.