Don't get sucked into the low fat nonsense (based on outdated 60's research, largely by Ancel Keys. Unfortunately, knowledge of the fallacies of his conclusions have not yet filtered through to "health professionals"). Your body is perfectly capable of turning sugar with 0% fat into swathes of body fat.
Avoid simple carbohydrates. Modern fruit juices and yoghurts are packed with simple carbs, unfortunately. Try to avoid modified corn starch or whatever codename they are using for it on the label. Get enough protein to keep muscle mass up and your metabolism ticking over.
Exercise (of course) and building some muscle (especially thighs) will get you to the point where you'll almost be struggling to keep weight on.
Oh, and supplement largeish amounts of vitamin D (Solal has a 1000iu D3 tablet that seems pretty good - take 2 or possibly more a day, but monitor blood D levels).
Excellent Post!
Controlling weight is all about controlling your insulin...carbs unfortunately make your insulin go haywire!