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Make sure the straps are loose--you can tighten them later if you need to. If you have trouble hooking a bra behind your back, you can hook the bra at the front and then twist it around your waist so the hooks are at your spine. The cups may feel empty or a little too big at this point, but that's ok. The band should be snug and low on your back. Pull all the soft flesh forward and up. Then hold the bra in the center between the two cups and jiggle it. Straps shouldn't be so tight they dig into your shoulders, but there should be no slack.  Does the bra pinch your breasts or make it look like you have four boobs? Then it's the wrong size.  Your nipples should be about halfway between your shoulder and elbow.  If your breasts are spilling out of the sides of your bra, you need a different size. Just because you are a 34C now doesn't mean you will always have those same measurements. If you've gone through a major body change or your bra doesn't feel right, take your measurements again. Get refitted if your weight has fluctuated more than 10 pounds, you've had a baby, started exercising regularly, or have completed hormone therapy.
Hook the bra so it is hanging around your waist. Lean forward and pull the bra up from the front only, sliding your arms through the straps. Lean forward and, using the opposite hand, reach into the bra and pull the soft flesh near your armpits into the cup. Check the fit and tighten the straps. Remember your bra size is not permanent and will change along with your body.