This Band-Aid for tennis rackets saved the day for me during a tournament when I was playing well with a favorite racket, but the strings were frayed and close to breaking. It’s a little tool that lifts the string and inserts and leaves behind a small piece of plastic that sits right at the intersection where the two strings cross, preventing them from sawing across one another and breaking. Extra plastic inserts are stored in the handle.
If you carry it in your tennis bag you can use it to extend the life of your strings, though obviously not indefinitely. It’s essential if you have only one racket and you don’t want to sit any games out.