Fitting a bra perfectly has 2 elements to it...
Determining your correct bra size
Correctly adjusting your bra for a perfect fit
Assuming your bra is the correct size and in reasonable condition, this is how to check whether it needs adjusting...
Lift up your arms. The bra should stay put around your body. If it rises it is too big around the band, so tighten the band
The center of your bra should sit flat against your chest
It should leave no strap or band marks on your body at the end of the day
You will need to adjust your bra after you have bought it, before its first wear, and possibly after washing.
Make the following adjustments while standing in front of a mirror...
Slip your arms through the shoulder straps (if you're wearing this bra for the first time extend the shoulder straps first)
Lean forward slightly and let your breasts fall into the cups.
Fasten the bra on the middle hook and eye. As the bra gets older (and after wash and wear) it may need to be fastened on the tightest hook and eye. If you put on weight after you bought this bra it may need to be fastened on the loosest hook and eye. You should be able to put your hand under the back of the band and still feel comfortable
Adjust the straps so your nipple comes roughly half way
between the top of your arm and your elbow (if you have a low and heavy
bust you will find your nipples are closer to your elbows than your
armpit, but make sure the straps are adjusted as high as possible).
You should just be able to put 2 fingers under the straps at the shoulder and still feel comfortable. If you have un-even shoulders or an un-even spine you can adjust the straps to different lengths
If there's a wrinkle in the cup use your hand to adjust your breast into the cup
The lifespan of your bras will depend upon things such as how many you have and how often you wash them. For example, if you have 3 bras that you wear during a week they'll typically last 9-15 months.
The underwire cracks!
When you're not wearing the bra and you pull on the strap and the strap length increases
When the color starts to look grey!
When you keep having to keep tugging the diaphragm band down to keep it in place!
When the elastic shows signs of wear