Listen closely: the band does 90% of the heavy lifting. Not the straps—the band. For saggy boobs, a too-loose band is the enemy of everything good. You want a band that fits snugly on the tightest hook when you first buy it. (I know, it sounds masochistic, but your boobs will thank you later.)
Pro tip: test it by raising your arms over your head. If the band stays glued to your ribcage without riding up, you’ve found a winner. If it creeps toward your neck, toss it in the “hate” pile immediately. No negotiations.