To keep your bra straps from slipping off of your shoulders cut a 2 inch rectangular piece of rubberized shelf and drawer liner and place it on your shoulder under your bra strap. It works!
Try nipple pasties on your shoulders to stop a loose bra strap from slipping They are flesh coloured with a thin gel to give comfort and grip. They are cheap, reusable and self-adhesive.
Use a Band-Aid to hold your slipping bra strap in place. Simply place the
Have you seen the mini-infomercials for the round plastic bra strap holders to allow you to convert your bra for inward-facing t-shirt straps - so that your bra doesn't show? I hate that.
To keep bra straps hidden, I have recommended the double stick tape (sometimes called "fashion tape" or such) between the dress and bra strap. A better idea, that is more re-useable, would be to sew in the oval spring clip type hair clips (child sizes) with a loose tack (so it moves a bit).
Hook a paper clip onto the back of your bra straps to hold them together. That way, the straps will not go down your arms.