Here's some tips that I thought were really helpful...I read them in a magazine.
When you're dieting, don't think about how many times you've failed before. Don't allow yourself to imagine you failing at your diet. Instead, "pretend" that you are that person that keeps going. Act like a person who sticks to the plan. Also, don't think about the reasons why you shouldn't quit, but think about the reasons why you should keep going. For instance, don't think, "if I quit I'll gain weight again." Instead think, "If I keep going, I'll feel great about myself and the way I look." Think about your own reasons to create a new lifestyle. Don't do it for anyone else thinking that you should look good for him or her. Rather, decide on reasons why YOU want to loook good for YOURSELF. This will help you stay focused on your goals and not what others expect out of you.
Stay motivated! Remember, we can do anything we set our minds to do. It's all in realizing that we have a new lifestyle, a healthy lifestyle where exercise is fun, sitting around is boring, eating right is delicious, and eating junk food is gross.