Here's my eventual plan to not gain weight back. I really think I'll have to count Calories for the rest of my life if I don't want to gain it back (except when I thru-hike the Appalachian Trail).
I'm running for 40 minutes 5 days a week right now and I have so much more energy that I find myself being a lot more active even when I'm not running. When I ever make my goal (a long way away--I started at 300 and I am down to 233 and I want to be 150), I plan to increase calories slowly until I find an amount I can eat and not gain weight. I am currently doing three meals a day with each meal just under 600 Calories. I figure when I get to my goal I can try adding a few more Calories to each meal until I hit an amount that ticks my weight up for a week. Once I hit that I'll drop back to what I ate the week before and see if that will help me hold my weight. I feel like exercise is the big thing that will give me a chance to keep the weight off and I hope to be running much faster and farther by the time I get to my goal.
Started at 300 lbs., now at 221, going for 150.