Make sure the student fully understands the material being discussed. If the student is falling behind, spend the extra time and resources needed to get her back on track. Partner with her parents to ensure the student is being supported during and after school.
Establish a conversational atmosphere in your classroom, where communication flows two ways. Allow students to tell you about their lives and feelings. Tell them about yours as well. Encourage your students to ask questions, even if their inquiries are off-topic. Feel comfortable saying, "I don't know," if you don't know. Create a non-judgmental environment, where students don't feel pressured to be perfect.
Make sure everyone has a turn. Calling exclusively on students who raise their hands to answer questions can create a harmful classroom dynamic where only a few students are fully engaged. Conversely, calling on students at random can create a strained, nervous atmosphere. Instead institute a system where students answer questions in a specific order. That way, everyone, no matter how shy, knows they will have to speak up at some point.
Praise right answers, and treat wrong answers as a teaching opportunity. Rather than saying, "Wrong," or shaking your head and moving on to another student, tell the student, "Not quite. Let's see if we can figure this out together." Eliminate the fear of being ridiculed or admonished for wrong answers. Always tell the student you're proud of her for trying. Help her to see learning as a joyful journey, rather than a system of punishments and rewards.
Eliminate classroom competition. Don't publicize grades, or treat any particular student, regardless of talent, differently from the others. If you praise one, praise them all. Help your student realize she isn't any better or worse than anyone else.
Facilitate bonding between the students by breaking them into pairs to learn more about each other. Rotate the pairs so everyone gets a turn with everyone else. This may help your shy student feel she's answering questions in front of a group of close friends, rather than a bunch of strangers.