Some of this will depend on the type of classes you teach and how often. Sean made some good points about keeping some main moves in each class. This will help some of your participants both feel comfortable with the moves and improve their form. Once form is perfected, then you can add on to those moves. If you are teaching a choreography based class, then you would want to keep much of it the same for maybe a few weeks to allow members the chance to learn and follow. Each class is different, so it may help to know what you plan to teach.
I do a combination of both–signature moves but I always like to challenge my classes with new moves for both an interest factor as well as muscle confusion. I tend to mix it up a lot.
Music is a big factor in motivation. I suggest having a nice collection of various play lists to keep participants interested and motivated.
Good luck and have fun! If your class sees that you are enjoying ourself, they will have fun also!