HijabiApprentice gave me some great advice once, when I was complaining about how my curly hair gets "used" to the shampoos and conditioners I use, and begins to look dull and lifeless--until I change brands, that is. She told me that this is because of buildup on my hair, and the reason that using a new product seems to helps is because it's washing off the residue from the previous product. And if I clarified my hair, I would't have this problem (or need to change brands).
So, for the past few months I've been clarifying my hair with baking soda about once a week, and I am AMAZED at how much better, fresher and *bouncier* my hair is! Here's how I use it, although there are other methods that are used: I take a small handful of baking soda, squirt some cheap shampoo into my hand along with it, mix it together and wash my hair with the mixture. Then, I rinse and wash again with shampoo only. When I condition afterwards, my hair absorbs the conditioner much better than it had before, and as soon as I step out of the shower, I can tell the difference in texture. My hair is sooo well moisturized and looks and feels so much healthier. So.. if you have a problem with dull hair, try this method of stripping off the buildup--it's cheap, and easy, and it works.
(image borrowed from www.bestofbakingsoda.com)