Your activity determines your level of support. Purchasing a low-impact sports bra is good for yoga or weight lifting; a medium-impact sports bra is ideal for hiking or cycling; and a high-impact sports bra is best for running or HIIT workouts.
The gentler the better. Either tuck your sports bra into a mesh lingerie bag before placing into the washer (set for cold water), or hand wash your sports bra. Regardless of how you wash them, hang your sports bra to dry to help it keep its shape and support.
You can avoid buying new sports bras prematurely if you keep yours out of the drier, as the high heat can ruin the elastic band that keeps your bra supportive. Stocking up on several sports bras you love and rotating through them will help keep your sports bras newer, longer.
Sports bras should be replaced every 6-12 months. Your sports bra stretches and contracts as it absorbs impact, which weakens the fibers within it that offer the most support. To maintain the best support (and coolest styles), shop for a new sports bra whenever you notice a lack in support, fraying or other signs of wear and tear.