What's a good way to lose weight fast and keep it off (about 100lbs)?

Life-Long Commitment. As a bariatrician, my role is to provide a patient with a life-long solution to lose and maintain weight loss. Based on recent literature, evidence shows, rapid weight loss via physician supervised programs are superior to slow gradual weight loss with commercial based programs. The key is following a patient for at least a year after weight loss. Maintenance is the hard part!
Make an appointment. Make a appointment to see me or a board certified bariatrician in your area.
Eat fewer calories. Successful dieting requires that a person stick to a plan for months or years, so easier goals & small milestones are good. He must be happy with the foods he gets to eat, so severe calorie restriction is usually unsuccessful. One can cut back on calories enough to lose 1-2 pounds a week, and he will be more likely to stay with that modest type of plan. One-two pounds a week is 50-100 pounds/year!
Read T Simeon's. Pounds and inches. It has worked for me to reset metabolism and keep weight off. It is controversial but works. Afterward one must follow healthy balanced diet and exercise according to guidelines.
Wait to lose Weight. If you rush to lose weight too fast, it is likely not fat you are losing at all. Your body prefers to burn everything else first before touching your fat. You must take the slow but steady path to increase your caloric expenditure, consume less calorically dense foods and to change your unhealthy habits for healthy habits. I suggest a coach along with a meal replacement program for success.