Actually XOR-ing the plain text with a key that is at least as long as the text itself is a very strong form of encryption. As long as you can keep the key secret, there is no way to reliably decrypt the encrypted text.

And of course you should never use the same key twice.

This is known as the one-time-pad encryption. Although it is totally secure and unbreakable, it suffers from the problem that you must find a way to provide the one-time-pad to the sender in a secure way. And as the one-time-pad is at least as long as the message you want to encrypt, you have only moved the problem on level "higher". If both sender and receiver can physically meet they can exchange the one-time-pad keys and the problem is solved. But you better not want to send more messages than pads you have!