I have the same problem as you, even my mom and hairdresser notice how dull my hair is.
I have done some reading up on it, and dull hair can be due to a deficiency is omega fatty acids. That's why people suggest eating lots of salmon and eating flaxseed or taking flaxseed oil. These foods are rich in omega acids, and a deficiency is them often will result in dry, brittle, dull hair.
Also drinking lots of water can help also. The thing with any diet or supplement is that it will take some time to see results because it will only affect the new growth coming in, since hair is dead, the hair that's already on your head will not be affected.
I would stick to a diet or supplement for a couple of months.
Many people here have said that flaxseed made their hair very shiny. I just started taking it again myself. I was never consistent in the past.
For a temporary fix, you can try hair rinses or even a clear rinse. HTH