I have very round shoulders and was constantly pulling up my bra straps. I just take a piece of 3/8 or 1/2" elastic and measure what I think is enough and then attach it to the back of my straps (one end to each strap) ( so it winds up in the middle to upper back of my straps. Yes, it makes it a little (and I mean a little) more difficult to put on your bra, but it works like a wonder. I don't have to pull up my straps anymore!! And I like that.
Also if you go to the "Soma" web site they have a bra that is almost like a racer back bra that is very comfortable (it hooks in the front) and no slipping straps!! Got to love it.