Bra strap cushions or pads slip onto your existing bra straps to provide comfort if they dig into your shoulders or can help stop straps from falling off your shoulders. Before you use them first make sure you have your bra straps set to the right length, most of the support in a bra comes from the band around your body, not the straps. The straps should be taut (not hanging) but shouldn’t be too tight, if you have an indentation left on your shoulders after wearing them they are too tight.
Bra strap cushions come in silicone or foam. Silicone has the added benefit of making them less likely to slip against your skin but you may not like how they feel. They are available in most lingerie stores and can also be bought online on websites like Amazon.
If your bra straps are at the right length and are still hurting when you wear them then bra strap cushions are the best solution. Look for why they are uncomfortable so when you next buy a bra you will know what to avoid. Thin straps are more likely to be uncomfortable so go for wider ones, also look for ones that are made of soft materials, some bras will even come with padding in the straps.
Straps that fall off your shoulders
Silicone straps can help with this but there may be a better method. If the straps on your bra have stretched out and are too long you could try a racer back clip, this brings them together in your back to secure them in place and will also get rid of some of the excess strap length. Or if you are comfortable with sewing then you could shorten the straps, this will mean you can still wear it as normal bra instead of having it racerback. If falling straps are a common problem for you, even with new bras then you could try a different style. Racerback bras are an obvious choice, or look for bras where the straps are set closer together in the back and the front (avoid styles like a demi cup which have straps that are set far apart).