How do you alter a ready to wear blouse so it won't slip backwards at the shoulders?

Question:The necklines on RTW blouses tend to fall backwards so that the shoulders ride on my shoulderblades and the front of the shirt rides up. I suspect there's an easy alteration for this but I don't know what it is.

Answers:
I used a small round velcro circle sewn onto my bra strap and the other side sewn to the inside seam of the blouse or shirt. If you don't want it permanent, you can buy velcro with adhesive on back.