At the beginning of the year, I assign each student a number and write it on his/her nametag. I have matching numbers on popsicle sticks in a can or cup on my desk. To avoid choosing the same students to respond all of the time, I ask the class a question and instead of choosing names I pull out a random stick. If that student is too shy or unsure of the answer, he/she may "phone a friend" to get help =)