It's always at the worst time.
You get up to speak in front of a crowd.
You have an armload of groceries.
And down your arm it goes.
Your bra strap. Again.
You tighten it. You tighten it again. No luck.
So what on earth is the solution?
Most of the time, slipping straps have nothing to do with the straps. Most of the time your band is too big.
Stick with me. Let's say the person in the pink bra is wearing a 32DDD. The person with the blue bra is wearing a 36D. Those bras fit them and the cups hold the same breast volume (cup size is relative to band size remember).
So when the brunette puts on the blue 36D, the cups "fit" (in that they hold the right volume), but the band is too wide and the straps end up right on the edge of her shoulders. She ended up in the 36D because she doesn't think her breasts are that big, so the DDD seems crazy, and 36Ds are everywhere- target, walmart. It's easy and cheap, but terrible to actually wear.
Sometimes you really do have sloping shoulders or other issues. In that case, finding the right size and style is important. For example, narrow shoulders will have a harder time with demis because the straps are wide set.
The lesson here is go see your local bra fitter. Book an appointment, share what you're looking for and let them help you. Slipping straps is an easy fix.