Your boobs should be filling out your cups, not overflowing. The cups should be snug and well-filled, not gaping, wrinkling or cutting in. If the cups are gaping, try going down a cup size. If the cups are overflowing, go up a cup size.
The band is what gives your boobs the support they need. If it's too loose, it will rise up your back. Your band should sit straight across your back, just below the shoulder blades. Just make sure that when you go down a band size, you go up a cup size, in order to keep the cup size the same. So, for example, if a bra feels too loose but you want to keep the same cup size, try a 32D instead of a 34C. If a bra feels too tight but you want to keep the same cup size, try a 36B instead of a 34C.
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 slipping off or digging in. All of our straps are adjustable, so make sure you adjust them to the right length when trying on the bra. If you can fit two fingers on top of each other under the strap comfortably, you have them adjusted perfectly.
The centre front
The centre front of your bra is the area in between the cups. It shouldn't be lifting up or pushing your boobs apart. It it's lifting up, try a smaller band. If it's digging into your boobs, try a bigger sized band (but make sure you go down a cup size, if you want to keep the cup size the same).