Racerback bras can be more comfortable than regular bras because straps stay in place and won't slide down the shoulders, as regular bra straps can. They can also provide better support, especially for full-breasted figures. You don't have to spend a lot of money on specialized racerback bras, though as it is a relatively simple matter to convert your own regular bras into racerbacks.
Lengthen each bra strap by about 2 inches.
Place a straight back chair in front of you with the back closest to you.
Hook the bra onto the straight back chair with the bra closure facing you.
Straighten the bra straps so that they rest on the “shoulders” of the chair back.
Pull the bra down the chair back until straps are only slightly slack.
Draw the bra straps in toward each other at the mid-back and pinch together with your thumb and forefinger.
Wrap the piece of elastic lengthwise around the two bra straps and safety-pin the ends of the elastic together. Do not pin the straps themselves. This will create an elastic “tube” that will allow the straps to slide freely.
Try the bra on. To do this, leave the bra unhooked and slide your head and arms through the openings simultaneously, position properly, then hook as usual.
The elastic should comfortably pull in the shoulder straps at mid-back for the classic racerback style. Tighten or loosen the straps further for a proper fit.
Slide the elastic tube to the proper position on the straps for a good fit.
Sew the elastic tube closed and remove the safety pin once a proper fitting is achieved.
For a completely adjustable fit, leave the elastic tube as is so that the straps will slide freely. Strap tension will hold the tube in the proper position with the bra on.
For a “fixed fit,” note the most comfortable position for the tube and mark the straps and tube accordingly with a washable marker. Affix the tube to the bra straps permanently by sewing in place.