Parents, my heart goes out to you...
Don't blame yourself - I know it is always easier said than done particularly when you are watching your beloved child fade away and feel powerless to help in any way. Always respond with love rather than guilt which is often unfounded.
Love your child unconditionally - eating disorders produce behaviours in your child that may not consciously nor deliberately endeavour to do. It is easy and even understandable to get frustrated by your child's ED related behaviours, but try your best not to project these feelings onto them as it may exasperate their guilt and low self-esteem attached to their ED further. Love them unconditionally through it instead.
Remind your child of who they really are - Your child would have been a whole different person before the onset of an eating disorder. Recovery means getting back to this person. Treat them for who they really are as much as possible and surround them with helpful reminders of their identity such as their hobbies and dreams. (My dad used to get me out of my bed by putting my favourite comedic movie on and watching it with me. This small gesture always turned a bad day into a better one!).
Be firm when necessary - Often when your child's life is in immediate danger you have to go against their wishes to ensure their well being. Of course they will protest against it(it is part of an eating disorder). However get them as much help as possible regardless if they want it or not. (I am now so thankful that my parents didn't take any of my 'crap' and got me the help I needed).
Look after yourself - Eating disorders are often a 24 hour job for parents and it is for this very reason that you need to take time out to look after yourself. You cannot best assist your child if you do not look after yourself first. Take the time to re-group - you derserve it!
(To my parents - I love you always & I thank-you)