I'm sure you are already doing these things, but if not then here are some ideas:
Pray that God would remove every obstacle in her life that would keep her from him. Pray specifically- you know your daughter so you know what has hurt her, what has hardened her heart and what has turned her away from God, so pray for those things to change. Ask God to work in those specific areas.
Also pray that God would send witnesses, bold, gentle true Christians who will be a witness of who He is to her, in action and in word.
Sing over her. It doesn't matter your voice. When you are weary of praying, but your heart is still crying out- sing. Praise God for her, thank Him for what He is doing ( even if you can't see it, trust He is working because you have asked and He hears you- that is a promise from His word and thank him) Also sing for her future, for the good things, for the beauty God has in store for her once she turns to Him.
This is a battle, not won in a day, but over a period of months, often over years. So don't despair and don't forget to rejoice in what God is doing and know that God has given you a powerful weapon- love. And as a mother you have a special role in her life, regardless of whether she can see that. Ahh I must go, I will pray for you today!