I found this idea in an old Australian Stitches magazine.
To stop dress/top straps from slipping off your shoulders, insert elastic into the straps when making your garment.
Make sure you only make the elastic slightly shorter than the straps or else they will really dig in!
I used clear elastic in my Kwik Sew top here. The magazine demonstrated with regular elastic.
Either way it stops those pesky straps slipping off all the time and makes it easier to hide your bra straps underneath!