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While choosing your strapless bras, choose at least one bra that is one band size smaller than your original measurement. Because a strapless bra does not have straps, you need to compensate for the lost support. You can compensate for this by having a tighter fitting bra, but it should not be so tight that it is uncomfortable. Assess if your bra fits correctly by examining the placement of the bra on your body's frame in the mirror. Your bra should center and lift your bust where it is midway between your shoulders and elbows. There should also be one inch of definition between your breasts.  Move around the fitting room and see if the bra stays in place. You don't want to buy a bra that you will be having to constantly readjust. You can ensure that the bra you choose fits the top or dress that you will be wearing with it by making a pattern or bringing a similar top with you.  You can make a pattern by drawing the outline of the top or dress that you will be wearing with a pencil on a tank top you have at home. When you are in the fitting room, try on the bra over the tank top with the pencil marks. If the bra goes over the boundaries made by the marks, then that bra will not work.   You can also bring a top or dress that is similar to the one that you will be wearing. Try it on with the bras that you have picked out to ensure that everything is covered.
Try on multiple bras. Assess your image in the fitting room mirror. Bring a similar top or pattern to shop with.