Most conditioners are designed to deposit a thin film of conditioning agents on your hair to reduce friction and add softness. These ingredients do not rinse completely away, however, even with the next washing. If you have hard water, it's even more difficult to remove the residue.
To remove conditioner buildup, use a clarifying shampoo formulated with EDTA that can eliminate product-trapping minerals. For better results, use distilled water when you wash.
Not everybody experiences build-up, but if you do, you can also rotate plant oils and moisturizers like glycerin, honey, pureed banana and aloe vera gel with traditional conditioners. Washing with baking soda or using a diluted vinegar rinse can also help remove conditioner build-up.