To help ensure that you call on students fairly AND randomly, use a random name picker. I've been using the name picker from Classtools.net. It's free and incredibly easy to use. Simply select "Random Name Picker" from the right side, enter your names (one per line), and save. It's that easy.
Click the disk to save it, or the gear to embed the Name Picker.
And did I mention it's all free? It's a great way to choose students in class - and it's fair. Give it a try and see what you think!