Sometimes all it takes is a smile. I have found that kind words can make all the difference in the world for students who lack self-confidence. I believe in praising student's small achievements especially from those who don't have many. Giving positive feedback, being careful on word selection, and having conversations on how to improve is beneficial while complementing them on their accomplishments is something else I practice in my classroom.
I will also purposely call on students to answer questions that i know they already know the answer to and give opportunities for students who normally wouldn't speak up in class to talk about something they are interested in or know a lot about(i.e. sharks, dinosaurs, bey blades).
Open communication with parents, speech teacher, resource teacher and/or intervention teacher is very important for student success and for that student to know their is a team working together for them. In some cases I have even created modified work and assessments for students to see an immediate response that they are "smart" or they "can do it" to gain confidence. Lastly assigning buddies and/or doing partner and group work with academically strong students often gives students that lack confidence a friend to discuss ideas with and be a part of team that is presenting the "right" answer.
School wide we have a motivational speaker who comes and talks to our students about how great they are and teaches them chants on various topics like test taking skills and bullying. His name is Kevin Bracey if any one is interested.
Technology is a huge help when it comes to student confidence. Any educational video games that are engaging make learning fun for kids. My favorites are iRead, a reading program where students can earn badges, awards upgrade their avatar. ABCya that has math and language arts activities where students earn points. Imagine Learning is language program that takes students through new and exciting levels while building vocabulary.