I'm trying to do the same but in the form of muscle so I am lifting weights and trying to eat more but it's not easy.
The whole trick is to take in enough protein and calories in a day to gain weight. If you want to gain any, you need to eat more then your burning.
Have 3 meals and 3 snacks all day, eat foods that are high in protein and low in saturated fat, like lean meats, nuts, egg whites, and low-fat dairy products. Have plenty, or your body will eat what your trying to gain. In my case the body will eat the muscle I'm trying to build, which it already has. I had a little set back because I didn't eat enough for a couple days.
You should also exercise like walking, jogging and lifting so the weight you can gain is lean muscle and not all fat.
Great thread you started here by the way. Like to hear what other people have to say on this topic.