Spam only occurs when the spammer is able to get your email address. They have a variety of methods for doing this, some legal and some not.
Either way, the result is the same. You have become a spam victim. The single most important action that you can take to prevent spam from occurring is to keep your email address off of those lists.
There are a number of ways that this can be accomplished, starting with never giving out your address to anyone whom you do not know or are not conducting business with.
You wouldn't broadcast your phone number over the internet, so why is your email any different? Be careful when putting your email address into directories or other form of printed lists.
Those directories could end up on the internet giving millions of people access to your email.
If, for example, you are a member of your alumni association and they decide to build a webpage with that information, your address has been exposed however unintentionally.
Make sure of the privacy practices that are in place with any organization that you give your email address to.
Do not give your email address out willingly over the internet. Your email address is often required when purchasing goods.
If you are willing to give them payment information, then you should feel comfortable giving them your email.
However, the fact that they will process the sale in accordance with law does not mean that they won't sell lists to spammers.
The sale of goods over the internet is regulated and enforced much more stringently than address list distribution. Often it is just too difficult to prove.
The site may ask you if they can share your information with their "partners" or "affiliates". These are simply companies to whom they sell information for the purpose of advertisement - spammers.
This box is almost always automatically checked for yes, so make sure you uncheck it before submitting your order.
In a forum situation do not allow others to email you via that site. They can simply post a response on the site and you can check it at your convenience.
Keep your email address hidden and not viewable to others when setting up the account. Spammers will try to join mailing lists for the sole intention of harvesting the addresses of other members.
If all else fails, create a junk email account. This account can be used to fulfill the requirement while keeping your real email address secret and off of any lists.
Do not respond to any spam that you receive. This includes following the opt-out link at the bottom of the page, sending irritated replies, or doing any business with a company that sends you spam.
They have already shown their willingness to ignore your privacy, and in most cases, the law. Responding only confirms your email address as real and attached to someone who reads their mail.
You will likely continue receiving spam from that company, and additional spam from other spammers to whom they sold your address.
Understanding that the reply only confirms your identity and keeps you on those distribution lists is a key to solving your problem.
Likewise, do not use auto-responders with your email. Auto-responders are away messages that automatically let those who are sending you email know that you are unavailable.
This is very convenient if you are going to be gone for some time. Unfortunately, this auto-response lets the spammers know that your email address is active.
Finally, do not forward chain letter emails. Forwarded chain letters can contain hundreds of email addresses before you ever get to the body of the letter.
Spammers collect these email addresses and then build distribution lists. They may also try to identify the addresses of people with whom you communicate.
This would allow them the ability to sneak by any anti-spam software by pretending to be from someone that you know.