Your boobs should be filling out your cups, not overflowing. They should be snug and well-filled, but not gaping, wrinkling or cutting in.
The band is what gives your boobs the support they need. If it's too loose, it will tend to rise up your back. Your band should sit straight across your back, just below the shoulder blades.
The underwire in a bra is designed to shape and support your boobs. The wire should sit naturally underneath your breasts without pinching or digging. If you notice the wire digging under your arms, try a bigger cup. If it digs into your chest, try a bigger band.
Your straps should sit snug on your shoulders without the tendency to slip off or dig in. All of our straps are adjustable, but if they are still digging then go for a smaller band. If you can fit 2 fingers on top of each other under the strap comfortably, you have them adjusted perfectly.
The centre front
The centre front of your bra just in between the cups should again be fitting comfortably against your skin. It shouldn't be lifting up or pushing your boobs apart. It it's lifting up, try a bigger band. If it's digging into your boobs, try a smaller band.