If my bra is not supportive enough, should I tighten up the shoulder straps?
Although you do need to make sure that your shoulder straps aren't too loose, it's the strap around your back that provides most of the support for your boobs. If this is too loose, you won't get good support, no matter how tight you make your shoulder straps. Instead, your bra will ride up your back and you'll spend half your day yanking it back down into place.
Sound familiar? If so, check that you've got your bra fastened on the tightest hook. If you can't fasten your bra strap any tighter, you need to try a smaller back size. But remember, if you go for a smaller back size, the cup size will be smaller too, so you will need to increase your cup size to compensate. For example, if a 36DD rides up your back and doesn't feel supportive enough, try a 34E or a 32F instead – you'll be amazed by the difference.