With all the will in the world, I do have this problem, although not with sports bra, more likely with normal bras every now and then if I go to the field straight from work and have to lug stuff around. Sometimes gravity can't be defied with even the best bra!
I'd get yourself a sports bra with racer straps, or as a quick fix, take a hair band and grab both of your bra straps on your back together and put the hairband round both straps and loop through to tie a knot. Keeps them together and they won't fall down :wink:
I'd maybe suggest your straps are too long if putting your bp on is making them slip down.