First, Place yourself in your bra
Take the opposite hand from the side you are adjusting and gently lift the breast from the back to the front to place correctly in the bra. This movement will help you settle your breasts correctly into the cups.
1. Straps shouldn't do all the work.
The support comes from the band, not the straps. If your band size is too big, you'll end up tightening your straps to compensate, which will result in too much pressure on your shoulders. If your band size is too small, your straps may be slipping off your shoulders.
2. Check the coverage.
There shouldn't be excess spillage out of the cups. If you're falling out or seeing a bumpy shape under your clothes, your cup size is probably too small. Try sizing up for a flattering, more natural looking fit. If the bra isn't fitting smoothly against you, and there's a gap between you and the bra, try sizing down a cup size.