If You Have a bra with a right fit
to your breasts you will not suffer from falling of your bra straps of your
shoulders (check How To Take Your Bra Measures).
But in fact, sometimes that may be
happens from time to time which affect badly on your whole appearance. To avoid
that, try to shortening your bra straps upward on each side of the bra.
is didn't work, you need to wear a racer back (also known as a T-strap).
Another option is finding a bra with Straps which have a nifty elastic gizmo
attaches to both shoulder straps across the back.