Behavior contracts that describe appropriate replacement behavior consequences and rewards can really help students succeed, eliminate problem behavior and build a positive relationship with the students teachers. Contracts can eliminate the never ending battle of behaviors that begins when a student engages the teacher and the teacher gets hooked. Contracts can focus the student and teacher on the good behavior rather than on the problems.
It is important to realize that this contract is between not just the teacher and the student but also the parent of the student. Since this is a contract and the student will be signing it, the parents have to see it. It spells out the expected behavior, the unacceptable behavior, the benefits (or rewards) for improving behavior and the consequence for not improving the  behavior. This contract should be worked out with the parent and the child and is most effective if the parent reinforces the appropriate behavior, rather than the teacher. Accountability is an important part of the success of a behavior contract. Describing the behavior negatively (stop hitting, stop speaking out of turn, stop swearing) will focus on the behavior that you want to extinguish. You need to be sure that you are describing the replacement behavior, the behavior you want to see in it's place. You want to be rewarding the student for the behavior that you want to see, rather than punishing the behavior you do not want to see.
With the contract, it is important for students to be able to see that they are succeeding in overcoming the behavior. The start chart to the left is a great way for student to see improvement. This chart is organized by days of the week. 
If I were to use this in my classroom, I would only have the school week days. When the student performs a "good" behavior, I would place a star next to the day the "good" behavior occurred. Once that star is there, it stays there. When a student performs a bad behavior, the teacher doesn't take that star away because it defeats the purpose of seeing improvement. Instead, that student will not receive a star. 
Another thing to keep in mind, is time of rewarding. The student needs to be able to determine what behavior was rewarded with the star. So in my case, I would talk to the student after class and give them the star after class. I would not do this in class because it gathers attention from other students. 
"Behaviorism is a world view that assumes that a learner is essentially passive, responding to environmental stimuli.  The learner starts off as a clean slate (tabula rasa) and behavior is shaped through positive reinforcement or negative reinforcement. Both positive reinforcement and negative reinforcement increase the probability that the antecedent behavior will happen again. In contrast, (both positive and negative) decreases the likelihood that the  antecedent will happen again. Positive indicates the witholding of a stimulus. Learning is then defined as a change in behavior in the learner." http://www.learning-theories.com/behaviorism.html