Carry spares duh!
This is a surprisingly not weird thing, most of my ex-girlfriends do this because it's easier than trying to hook it up behind your back, especially since the bra strap goes right between the area where you find it difficult to do yourself. Though my last girlfriend got incredibly lazy and pretty much every morning simply got me to do her bra up. That was a little weird because frankly I am male, I wanted to be undoing the bra an pulling her back in to bed lol.