Stretchy band inside of dresses...

Are you talking about a built in corset (not a laceup) of some sort?

Here is a photo of what my dress had inside. It came already built in by the dress designer. I'm not sure adding one would be a good substitute for taking the dress in though depending on the style of your dress.

My dress has two of them, there like the straps of a bra, one is in the chest area and one is in the waist area, I'm being told that they help keep my dress in place, my dress is strapless so it keeps it up so I don't have to keep pulling it up

I'm not sure what they are called but I also have 2 of them. The seamstress told me that they help hold your dress up if it's strapless so you aren't constantly pulling it up all night. Mine has 3 different hooks like a bra.

Mrs.O I love that! I wonder if my seamstress can put that in my dress.

This is my dress

http://www.davidsbridal.com/Product_Mikado-Ball-Gown-with-Beaded-Appliques-CWG436_Bridal-Gowns-Shop-By-Designer-Oleg-Cassini

I just feel like it gives me hips and I have no waist definition.

I guess that thing I wanted won't work since it's supposed to keep the dress up haha.

What you are referring to is a waist stay. It can be elastic, attached to the side seams of the dress lining, but is more often made of petersham, a type of ribbon, with an adjustable hook

http://gorgeousfabrics.com/blog/2012/04/05/the-what-and-why-of-waist-stays/