When choosing a special occasion bra, follow this rule of thumb: take the outfit with you when you try on the bra. There is nothing more frustrating than buying a strapless bra, putting it on at home to find that its got too much or too little coverage for your outfit.
If you are unable to take your special occasion outfit with you when you are purchasing the bra, then take along a similar style t-shirt or blouse in a similar fabric. Take note of how the bra looks through the fabric. Check to see how much of the bra will be showing. If possible, purchase two different styles so that you have some options.