La Leche League Strap-ees Style #79. $7.50.
La Leche's Strapees are handy little silicone pads that fit under your bra straps at the shoulder. Strapees prevent your bra straps from slipping down, they also help distribute weight to prevent bra straps from digging into your shoulders. They're easy to slip onto your bra strap and cannot be seen under clothes.  
Only in Beige 
Made of 100% soft silicone.