While I now realize the story I found is not the same one, I still feel like it deserves an honorable mention. Warning: the story is fairly dark.

www.foreverchanging.org/bb/viewtopic.php

I have successfully done this before (and I think others have said so on another posts). It is similar to coke and mentos as you might imagine. The main differences include:
-The reactions seems much more powerful, so be careful.
-The taste is awful, even when you mix the baking soda with water.
-May be able to let the baking soda fall to the bottom of your belly and thus the soda will expand your lower belly more (unsure about this though).

I honestly find coke and mentos almost as good without the nasty taste, plus it seems safer and more controlled.

I wouldn't waste any money on actually buying weight gain shake mixes. The difference between those mixes and a milkshake is more protein and more vitamins, a lot more vitamins.

Take your daily vitamin pill and break out the blender! I'm sure you can make your own mixes much more fattening.

...Although looking back at the comments it looks like some people are already making their own.

Loved it! Not exactly on my scale for girls...but the sound effects just added to the idea of a girl getting huge so quick!

Mod Edit: This isn't a post by the original poster, but since this is currently the first post in the topic a link is being added here.

supersquiggles.deviantart.com/art/That-s-You-Animated-197157441

Over 9000! ...whatever weight will make that shirt rip!

I seem to like a smaller scale than most: 170-220.

Stuffing with food seems the most satisfying but there are definitely other ways that are much easier. If your willing to try other things then bloating with coke and mentos then right after bloating with an air pump seems to have the best/easiest results. It seems she may not be very experienced, thus she may not bloat to a very big size...at first.

I know I don't want to bring Facebook into this, but some posts should be thumbed up. There's some good advice here.

All I know is that almonds are supposed to help lower blood pressure. Eating them should help with staying healthy and gaining weight.

I'll second that. Anything with lots of high fructose corn syrup is a great way to get a soft belly fast.