1. Put the bra on and buckle it on the least tightest hook. Although on the loosest hook, it should feel firm and tight without digging in. Slide two fingers in between the band and your skin. If you can fit more than two, its too loose and you will need a smaller back size. 80% of a bras support comes from the band. 

2. Bend over slightly, slide your hand into the cup and pull your breasts into the bra properly. ( DO NOT PUSH THEM INTO THE SIDES OF THE BRA) This ensures that all your breast tissue is sitting in the cups properly and assure a good fit. 

3. Adjust the shoulder straps. This can make all the difference! Tighten them a little if needed however there shouldn't be any pain or red sore marks. 

4. Make sure the band is not running up and is horizontal around your body. If it is riding up , then the band is too big. 

5. The middle of the bra should be sitting flush on your body for maximum support. If it is not sitting flush, you may need a bigger cup size. 

6. The cups should cover all your breast tissue ( certain styles are the exception) , without any spillage from the front or the sides. ( side boob) If there is spillage, you will need to go bigger in the cup size.