|Here's an easy tip to help keep those bra straps hidden when wearing tanks or sleeveless garments.
Cut a strip of 1/4" wide ribbon (twill tape would do as well) the width of your shoulder seam measurement. Fold over and finish both ends. Secure one side in the armhole at the top of the shoulder. You can easily do this when you finish the edge, whether it's folded over and sewn or bias trimmed. With facings you might want to hand-tack.
On the other end, sew the snap. Then secure the other half of the snap to the shoulder seam allowances near the neckline. Measure first of course, you don't want to make it too short.
That should keep those straps in place. It also helps keep a wide neckline from slipping off your shoulders.