Here are several common causes and a few unexpected ones as well:

If the band is too loose, the back will ride up (since gravity is pulling the front down). At some point the cup may still be anchored under the breast but continue to tip forward. This can create slack in the shoulder straps allowing them to slip.

If the straps are too tight, yes too tight! they can slip. Imagine you are wearing a bra and you feel like the cup isn’t supportive enough so you tighten up the straps trying to lift the breasts by sheer force alone. But gravity rebels and the weight of the breast plus the tightness of the straps create a substantial amount of downward force that then pushes the strap down into your skin and right off your shoulder! Or worse if you get the strap tight enough to stay put will bearing all the wieght your shoulders may actual form deep and possibly permanent grooves from the constant pressure.

Shoulders are round, some more round than others. Perhaps the style of bra is not suited for the particular shoulder type. Wide set balcony straps are great for people with average to wide shoulders or square shoulders. Women with really round or really narrow shoulders would probably do better with a “center-pull” strap (that means the strap comes up directly over the center of the breast and therefore more centered on the shoulder as well). Center-pull straps are most often found on full-coverage or full-cup bras like Fantasie Smoothing Full Cup or Chantelle Amazone.

For women who really can’t stand the strap struggles try a racerback or convertible bra so that the straps are actually pulling diagonally across the back making it much harder for them to slip down. You can also use a product like The Strapper or The Hook Up (you can find them under Intimate Solutions on http://www.alamodelingerie.com) to convert a regular bra into a racerback. Both products are a great way to make use of your existing bras if they otherwise fit.

There are other unique nuances that create strap dificulties but check the fit of the bra first because you are not alone in the shoulder strap battle.