Why I want to do it is because I have a BMI of 18 and even though I'd like not to worry about what society deems attractive I do have to live in society.
I do take your point on having to keep up the change in dietary habits for the rest of my life, but then again obese people have to change their dietary habits permanently.
I probably do under eat but one of the positives of that is, according to studies on mice, I will live significantly longer
If you're dead set on doing it, then, forgive me for trying to convince you otherwise.
Here's a list of what are, in my opinion, quality foods to consume much of:
Cottage cheese
Yogurt
Legumes: lentils, beans (black-eyed, Navy, lima, etc.), quinoa, split-pea (great soup!), nuts like macadamias, Brazils, Hazelnuts, almonds, walnuts, pine nuts, etc.
Grains: Rice, pasta, bread, etc. (are there any other grains? =P)
Potatoes
Fruits/Veggies: Papaya, raisins, celery, pineapple, apples, avocado, cabbage, kale, radishes, onions, spinach, bananas, berries, kiwi, melons, plums, pears, nectarines... Why do I mention these things? Because one, they're healthy. Two, I like to think of my metabolism as a furnace. When you're not hungry enough for a full meal, you can stoke it with fruits/veggies.
And obviously you can combine things, like, say, peanut butter on celery, or seeds (pumpkin, sunflower, etc.) and granola and fruit in your yogurt.
Weight gain is SOOO simple, people tend to over-complicate it.
Weight gain (and loss) comes down to nothing more than calorie balance
Not every calorie is not created or metabolised equally.
Eat as often and as much as possible.
I feel this is terrible advice, advice that will win the battle but lose the war.
What I did: Shrink your stomach! Eat only when you're hungry, and eat slowly and with mindfulness, and as soon as you're not hungry, stop. Then eat a little more, etc. This also has the nice side-effect of making your abdominals more noticeable.
After a week or so of this (I was also working out), my metabolism got much faster and I was able to eat more often, yet still healthful.