A Basic Bralette
What it does: If all of your bras are acquaintances, the basic bralette is your bestie. Typically unpadded, seamless, and either made of cotton or spandex, it's one of those everyday bras you always turn to because it's supportive yet still comfortable.
When to wear it: The question with this bra isn't when to wear it but when not to. Resist the urge to wear the same one every day. It's true that bras should have at least a day off between wearings as body heat tends to relax the fabric.
When to replace it: This is the one you actually need to replace most often. If you wear it several times a week, you might get four to five months out of it before it loses its fit. Luckily, you can get these on the cheap side, so stocking up often won't be so frustrating.