Visualize the healthier, lighter, pain-free, more agile you. Start with the end in mind.
Remind yourself regularly why it's important to you and why you're doing this for your benefit.
Get a cheerleading section that you're comfortable with to encourage you (with affirmative messages).
Set goals in small increments -- doable increments, maybe 5 pounds at a time so that your subconcious will accept the notion of your losing weight at a reasonable pace.
Tell your goals to people who know you and who care about your well-being.
DRINK LOTS AND LOTS OF WATER.
Acknowlege that it took a long time to gain the weight, it will take some serious dedication to lose it, but it's well worth the effort.
Now if I could only follow all of this sage advice...