This rule applies to breast forms as well. An external breast prosthesis or silicone breast form should fit as close to the chest wall as possible, allowing a properly fitted bra to hug the silicone breast form and keep it securely in place. This is not possible with a heavy breast form or an ill fitting bra / mastectomy bra.
We came across a website that is pretty cool, it is a lingerie site HerRoom.com. The great thing about this site is you can fill out a questionnaire called “Know Your Breasts”, and they will send you a report suggesting styles and brands of bras that will help address your needs.