thesolution is Strap Trap: racerback bra clips
Bra Clips are a simple way to hide your bra straps when wearing racerback tanks, sleeveless tops, or any dress that exposes your bra straps to the world. Virtually invisible, flat and flexible, using these means that you don't constantly have to tug at your slipping straps. Once attached, Strap Trap bra clips stay exactly where you place them and do not slip down or fall off. As an added bonus, Strap Trap provides additional lift and support, giving you a little perkification along the way.
Using Strap Trap is easy:
1) Before putting on your bra, loosen your straps slightly and attach Strap trap to both straps as depicted on the package, placing the strap under the center clip and over the top and bottom pieces.
2) Slip your bra over your head and fasten the hooks in back.
3 Strap Traps: Black, White and Clear. 1.75 L x 1.1" H.