I have had several very shy, timid, dogs, and the best thing you can do is be patient with her. Project a very calm, gentle, and confident demeanor around her. Dogs are very sensitive and will pick up frustration or impatience from you. I would talk with her in a calm, yet cheerful tone, and gently encourage her to play and explore and give her tiny treats when she is being brave.
What I've found has also helped me tremendously is to support her transition with flower essences. I've used Spirit Essences
Spirit Essences - Holistic Remedies for Animals, and used their "Foster Care Remedy", "Self Esteem" and "Stress Stopper" with wonderful success.
But remember that it may take several days, even weeks for her to build her confidence. Just project love and patience with her, and she will return it back to you ten-fold!
Good luck, and keep us posted!