After doing a new activity with your class, have them write an evaluation about it. They should tell you briefly if they liked the way it was taught, if they learned from it, and suggestions for making it better. It's easy to continue teaching lessons the same way every time--some student input might help your future learners to understand things better or easier. You can get a lot out of the way they act during the lesson as well, but sometimes it's helpful to have it written out.