You do not need to wash your bras every time you wear them. You can wear a bra three or four times between washings as long as you do not wear the bra on consecutive days. The elastic needs at least 24 hours to recover between each wearing. If you sweat profusely, or if the weather is particularly sticky, wash your bra after each wearing. Perspiration can cause more damage to the bra than washing, so frequent washing will keep your bra in better condition.
If you have the time, washing bras by hand is the best way to keep them in good condition, as bras are easily tangled and snagged in washing machines. You should also hang your bras to dry since the heat from the dryer breaks down the elastic.
If you must machine-wash your bras, make sure they are fastened and placed in a mesh laundry bag. Use the delicate wash cycle with cool water. Add a gentle detergent recommended for delicate fabrics.