I would get a fecal count done and have her teeth checked by your vet, first of all. You can also have the vet run a CBC to make sure there's nothing else going on. Everyone else's suggestions are great, but I've also had tremendous luck with adding in cocosoya. As a plus, it gives them a fabulous coat. You can buy it in oil form or in a pelleted version from SmartPak with a few other things added in. 

A warning: my critter suddenly developed tons of energy after I put her on it—I'm not sure if it was due to the cocosoya (I mean, it's an oil) or the change in weather, but she's still feeling her oats at almost 22. She's still gaining weight steadily after two months on it, and she's only on 1/2 a cup.