Dull hair that lacks shine is usually associated with dryness, but using the wrong products for your hair type can dull the hair even more and exaggerate dullness further. Using products that overload the hair will also attract more dust and grime from the atmosphere than normal and add a film on the hair surface, thus inhibiting light reflection.
It may sound obvious but for best results you should regularly cleanse your hair with the correct shampoo for your hair type and pay special attention to your conditioner and application.
Damaged hair, such as chemically treated and coloured doesn't generally shine as much as natural hair as this process can cause the hair cuticle to be raised. If you use a product such as System Professional Fluid Crystal it will assist the cuticle to lay flat thus increasing smoothness and allowing light to reflect off the hair shaft.
Due to its flat surface straight hair has more of a tendency to shine greater than curly frizzy hair. Shine sprays such as Tigi S Factor Chic Shine and Tigi S Factor Serious Conditioner are great to promote shine for all hair types - particularly curly, frizzy hair. Bedhead Headrush is a great lightweight spray that can be used during styling to instantly add sheen to any hair type at any time.