I've had unsuccessful attempts at weight loss so I know exactly what it means for motivation to peater out. I'm feeling so pumped this time around (I start tomorrow) but I think it's because I planned ahead for a week. I organised my exercise program and paid ahead (I'd hate to loose the money now!), I wrote down my goals for weight and exercise, I joined this site and I just planned and shopped for my meals for the next 2 days. I've never been this organised! Plus, I've made lots of public claims to my friends so I'd hate to back out.

Hope this can be helpful. Maybe just take some time to come up with a solid plan.