The key to the achieving the best fit of a wedding gown is the bust area.
With a strapless gown the bust needs to be fitted tight, but breathable to keep the gown from pulling and slipping down through out the day.
The first step is to purchase a corset or well supporting strapless bra.
Corset bra's will give the best fit and support while contouring and shaping the waistline as well. Bra cups can be sewn in a gown to fill out the bust-line but they do not support the bust. A good alteration specialist will have several techniques to use when fitting a heavy strapless gown.