How to lose 100lbs in about 6-8 months?

I just got engaged on 12/25/08, and we've set the date for 2/13/2010. I know it looks like I have 1 year but I have to be ready to buy my dress in like 8 months so I can use the remainder of time for any alterations (hopefully only taking-in and no taking-outs). Also, we're going to San Antonio to Hyatt Regency Hill Country Spa and Resort for our daughter b-day,and I want to look at least decent in a swimsuit. FYI. If I can steer away from pills and fad diets that would be great!!....Thanks!