If you apply moisturizer to dirty hair or hair that is covered in product build up, there is a slim chance the moisturizer will reach the hair's cortex. Your hair doesn't have to be perfectly clean every time you moisturize but it should not be in desperate need of a wash.
Focusing on the roots of your hair instead of the ends and vice versa. Don't concentrate solely on your ends and neglect the rest of your hair. All of your strands need proper moisture. Take the time to make sure all of your hair is thoroughly coated with a moisturizer and sealant.
Do not use styling products as moisturizers. Styling products are used to hold or set a style while moisturizers are specifically designed to give moisture. Yes, there are some styling products that also act as moisturizers but most do not. Check the label and ingredients before usage.
Moisturize your hair on a daily basis. Even if your hair does not feel dry to the touch, it can still be in need of moisture. If you are experiencing breakage, you may need to up the amount of times you moisturize per week.