We are not sure if wearing a rubber band around your testicles will make you last longer. We do know that men sometimes put something around the base of their penis - often referred to as a "cock ring" - in order to keep an erection longer. However, we do not recommend doing either of these things.
When you are sexually aroused, blood rushes to your genitals. If you keep the blood there artificially for too long, it can be dangerous and might cause blood clotting. Sometimes the experience of being aroused can be distracting, and can cause a person to pay less attention to pain and discomfort that signals the body is in danger.
While we recommend against it, if you feel you must experiment with such things, we urge you to be very careful. In addition, if you are concerned that you are not lasting long enough during sexual activity, we want you to know that there are other ways to deal with this. If you are newly sexually active, you will last longer as you become more accustomed to having sex. If this is something that concerns you, try talking to a doctor or someone else you trust about it.
Finally, if you are sexually active, we hope that you are practicing safer sex. While some people do not like that condoms dull sensation a little bit, the muted sensation can actually be useful in prolonging an erection.