When making this hinge use a crayon to mark the center line or hinge line on both sides of the hinge. This helps keep the hinge centered by giving you a reference mark, but more importantly seals the hinge line keeping the glue from soaking into the hinge making it stiff.

Z hinge this is probably the best write up I have came across yet on Z hinges. http://www.rcgroups.com/forums/showthread.php?t=906735