In this tutorial, Creativecow's Mylenium demonstrates various lens flares and how to create them. The main focus of this tutorial is not the lensflare itself - there are enough plugins out there, that do a far better job than even the most complex setup we can cobble together. Much more is it meant to again teach you some tricks to work efficiently with expressions and breaking down projects into workable parts. Lensflares lend themselves to illustrating the benefits of expressions because they involve lots of layers and effects.
Click on the link above to view Mylenium's tutorial.