A Compilation of gaining tips, gathered from the Forum.
These will be updated as new tips show up, so check back often to see if we have something new in store.

If you're really underweight you should consult a physician, so you know what kind of weight to gain, (muscle/fat), and how much of it to gain, and how to do it. The internet, amazing as it is, does not have a PhD...

Allmost every teenager goes trough a time in which his metabolism is fast, but it will eventually slow down. I can't say "next year it's slow", some people have it untill their 20's and some well into their 30's, and others don't have it at all.

Actually building fat fast depends on the persons ability to burn or slowly burn calories? we all have different weight gaining ability. Your system can not process easily huge amounts of food, it will absorb only what it can during digestion. You may just end up with a fools bloat that appears to be a bigger belly but really isnt` you want a honest gain that only comes from a reasonable duration of time. If a milk bottle only holds a half a gallon of milk? to continue pouring
more milk into it will simply drain down the side of the container.

You are going to have to drastically reduce your exercise and movement and drastically increase your food intake. A good plan is to make sure you are always eating. And not carrots, I'm talking about ice cream and other high calorie foods.

You need to eat more carbs than you burn each day (don't overdo it too much though). Also, limit your physical activity. Instead of going outside and playing soccer, stay inside and play a video game or read or book. But don't limit your physical activity too much. Just do some walking so that you can still gain while strengthening your legs to accomidate for the extra weight. Snack at every opportunity you get.

Eat a lot before you go to bed. I think that's one of the more common routes.

Too much sugar consistantly can make you sick and can also cause you additional health issues later on. Plenty of nice foods that just consist of carbs, doesn't need to be sugar loaded. Eating a lot of food generally at night helps anyway, no matter what the nutritional value or content.

Fast-Food 3 Times a week or so is a good gaining aid, but I defintely don't recommend doing it over a long period of time. Trying to stay a little more healthy rather then going all out is very advisable when gaining (Ignore people that say you have to be insanely unhealthy if ur gaining).

Ice cream shakes are a good way to gain fast, but they aren't good to do for a very long time.
All the dairy generally has mainly saturated fat, as well as excessive amounts of diary not being too good anyway.
But be careful with too much diary - If your sensitive too it (like me) it will give you huge stomach cramps and heaps of gas if you eat too much. I would recommend to just stick to just eating as much as you can... simple math... More Energy then you need will make your body store it.. if it doesn't end up being used this turns into energy reserves (fat). I generally stick with foods loaded with Carbs moreso then just Dairy.

Most weight gain shakes don't taste very nice and the predominnant ingredient happens to be protein, which does a lot less putting fat on the body the eating carb and fat rich foods does, so it won't make too much of a difference.

To get a ball belly, you also have to be lucky in the gene department, got mine from fathers side of family ! but definately gassy drinks like beer, pop, and eating after a night in the bar and going to bed with a full gut helps!
Although heavy lifting does promote a ball-shape belly. Don't do sit-ups or running. Single bouts of heavy lifting doesn't burn calories, but it strengthens abdominal muscles. That, combined with weight-gain builds a ball belly. Power-lifters can tell you that those men with "Mr. Atlas" physiques could never lift astronomical amounts of weight. Their mid-sections would burst under the pressure! The fat is needed to support the muscles. Remember...your mid-section is the centre of gravity. All you lift is supported there in your mid-section, including your back.

Water has no actual effect since its just that, its not going to make it any more jiggly. It could make it a bit more flexible if your capacity for water is relatively high. Usually though, water will just bloat you out and make you firm. Having a jiggly gut or a hard gut is mostly a matter of genetics.

Everyone is welcome to contribute to this Thread, if you feel like we left out something.