All costumes need some adjustments in order to fit right; a little sewing is normal after purchasing any costume to ensure the best fit.
Bra: Some ladies might need to take out padding that a designer has put in. This is accomplished easily by undoing a tiny bit of the lining and simply pulling out as much padding as needed. The neck and back hooks may also need to be adjusted in order to create a perfect fit.
Belt: The most common fit adjustment on the belt is to move the hooks. Usually the top hook is set so the belt is slightly smaller at the top. This helps prevent shifting. It is also possible to add elastic with a thread casing to prevent gaping at the top of a belt.
Skirt: If the skirt is a tiny bit tight, try letting out the elastic at the top a bit. If it is too loose, you need to make the skirt tighter. Accomplish this by either tightening the elastic that is already there or adding elastic yourself.