Here are some tips on how I grew my natural hair past shoulder length:
Protective styling: Tucking away your ends and keeping them from rubbing against your clothing. What usually happens is your hair gets to the point where it touches your shoulders and then it rubs on your clothing and breaks off. Try braids, twists, buns and ponytails to keep your hair off of your shoulders.
Focus more on moisturizing your ends. Use a moisturizer and then an oil or butter to seal. To keep your ends from breaking off, they need to be moisturized.
Healthy hair always starts off at the scalp. Be gentle with your hair. Yes, this means during your detangling session also. Always start from your ends and work your way up while detangling. If not, there could be some serious breakage that could be the cause of stunting your hair retention. Take care of your scalp. This means deep conditioning and applying moisturizers when needed.
Limit your heat use. Heat styling dries out your hair and therefore can cause breakage in the long run.
If you follow these tips, your hair should grow past shoulder length in no time.