Our little Pup is one of those dogs that has trouble keeping her weight down, and we have to keep her slim because of her hip dysplaysia. (Which also contributes to the problem, because she doesn't run and play as much as most small young dogs would.)
We keep her body shaved short so that we can watch her waistline, and we have her on a fairly strict diet. She's about 20 pounds, and she eats (total for the day) about 7/8 cup of Nutro Max Weight Control, and over 1/2 can of no-salt veggies (usually green beans). The vet told us that we could supplement the veggies as a stomach-filler, as long as she was still acting like herself, in terms of energy level. If she started to act lethargic or weak, that would be a sign that the veggie diet wasn't working, we were told. But, she does great on it - high energy; she act young and exuberent! She gets regular treats - we give alot of small ones in a day, rather than just a couple of big ones. If she has a high-activity day, we give her a little extra food to make up for the expended energy!
Anyway, this is just what has worked for us!