A Sports Bra
What it does: Even if you aren't athletic, you need to own a supportive sports bra to help prevent breast tissue damage. Even light walking can stretch and break down the ligaments that support your breasts, which leads to sagging, which leads to the need for more bras.
When to wear it: Often categorized by the level of impact and exercise intensity, you might need a different sports bra for different activities.
When to replace it: It's crucial that the efficacy of the sports bra is intact, so if you think it's not performing as it did when you first bought it, it's time to toss it. If you aren't sure, get a new one every six to eight months.