Include a few unpleasant tasks in your standard lesson plans. Include things like a quiz every Friday covering the material you taught during the week, or cleanup time at the end of every school day. Make sure that these tasks are essential for your class and teach your students something valuable, such as how to clean their workspace or the retention of information from your lessons.
Determine an alternate way for your students to achieve similar results for your unpleasant tasks, but through less obtrusive methods. Inform your students that if they take advantage of your alternate methods, they can avoid the unpleasant tasks you have planned. For instance, if your all of your students turn in all of their homework on time during the week, they will not have to take the end-of-the-week quiz. Alternately, give your students time to clean their workspace after each task, and let them watch television or go outside for the last few minutes of your class day, instead of having to clean their workspace after class.
Plan a few fun activities during your semester, with specific goals for your class to reach for in order to earn those fun activities. Include short-term activities as well as long-term ones. As an example, if the class average on midterm and final exams is a 90 or better, host a party in your classroom during a normal class day. Alternately, you can give your students two turn-in dates for major assignments, rewarding students who turn their work in early with a few extra points or free time on the playground during the following class.
Stay consistent with your reinforcement. If you need to make changes, change your system between semesters and refine your reinforcement with new activities and ideas. Avoid changing during a semester, after some of your students have already made the decision to benefit from your reinforcement. Remember that some students need time to get used to positive and negative reinforcement techniques, and changing during a semester may keep them from accepting your techniques.