Give a few volunteers a toothbrush and towel. Have people in the class say things that help them live the gospel or keep the sabbath day. For each thing said, spread some toothpaste on the brushes till its overflowing and then have the people brush with it. Their mouths will be overflowing with paste!
Then say "well, they have brushed with enough paste to last all week, they wont have to brush the rest of the week right?" and then have people compare it with the gospel they need to live the gospel everyday they cannot save it up just for Sunday.