I feel your pain on the whole gaining weight back thing. >.< I used to be so good about eating well and going to the gym but as of late I've gotten kinda really lazy and unmotivated.
Something that seriously helped me was weight watchers. Instead of just counting calories they base points on carbs/fat/protein/fiber and I've found that to be way more effective. Honestly, you don't even have to sign up for the program though. That's way too expensive. On some app stores they have programs that do the same exact thing for, like, $2. iTrackbites is what I'm going to be using.
As for fitness, the elliptical is my friend and I do a lot of weight lifting with less emphasis on weight and more emphasis on reps. I really want that nice butt, flat tummy, and Obama arms. xD
Are there any specific cosplays that you're using as motivation? I'm determined to get fit enough to cosplay Yoko Littner by the end of May.