You need to know the proper command, which depends on which list server manages the list you want to use. You also need to know the address to which you have to send the command. This address is called the administrative address, the address for administrative commands. Don't send them to the list address, where they may be distributed to all the subscribers of the list, and make you look like a clueless idiot. Instead, send them to the administrative address, where the list server program can carry out your commands.
Most list servers ignore the subject line of messages.
If your e-mail program usually adds a signature to the ends of your messages, turn this feature off for this message. For lists managed by a human being rather than a computer, just send a polite note to the list manager. Sending repeated messages will just tick the manager off. For lists that are managed by a Web-based site, like Topica or OneList, go to the Web site and follow the directions on the Web pages. Lyris lists also have a Web interface that you can use for subscribing and unsubscribing.