It is almost inevitable that as time passes, the fabrics in clothes begin to deteriorate. This is especially true of bras. The more you wear and wash them, the faster they fall apart. Arguably the worst part in the process occurs when the underwire rips through the fabric and starts poking out. Though there's no way to stop the wires from one day ripping through the fabric, but there are ways to repair bras and extend their life span.
Slide the underwire back into place through the ripped opening. Give yourself some slack and push the wire back in as far as it will go, leaving as much room as possible on the side you will be sewing.
Secure the ripped opening with some fabric glue for added stability. The underwire can slip out of the tiniest opening, and the glue will cover a larger surface area than the thread alone. Let the glue dry for a few minutes before proceeding.
Thread the needle and sew the area where the rip occurred. For smaller rips, you will need about 10 stitches and about double that for large rips. You may want to extend the sewing area around the rip itself for extra security.
Secure the thread in place with an additional drop or two of fabric glue. The glue should fall relatively flat as it dries, but if not, you can use a nail file to smooth the glue drops.