• Use any filters your ISP or email provider have in place
    Ask your ISP or online service if they provide any filtering, and if they do then take advantage of it. If it’s important enough to you, and they don’t provide that service, either switch or put pressure on them to add that feature.

    AOL users can enable filtering by changing their preferences somewhere online—under Marketing Preferences, I think, but it’s been a long time since I used AOL (and they didn’t offer those filters back then).

    Hotmail and some of the other web-based email services also offer spam filtering now, although I don’t know how well they work. GMail’s filters are quite efficient, so much so that I don’t think I’ve ever received spam in that account. I have learned to check for valid messages marked as spam, though.
    Caveat: if your ISP or other service uses a “challenge and response” (C/R) system to avoid unwanted messages, please don’t use it. C/R systems are broken, and make it much less likely that you’ll actually hear from legitimate contacts. This article offers a good summary of the problems with C/R spam filtering.

    Use the filters in your email program. Most of the commonly-used programs, like Thunderbird, Outlook, Eudora and Pegasus, can filter your mail for you now. Check cnet’s Download.com if your current email program doesn’t have filters built in. Even though I get hundreds of messages a day, it’s always easy to spot anything unwanted that gets that far right away—the spam stays in the in-box, and all the expected email is filtered to folders for lists, newsletters, correspondents, etc.

    If you run your own mail server, you can use procmail or Spam Assassin or other tooks to set up filters at the server level.

    Then there’s the not-free—but not expensive—service, SpamCop. They catch almost all the spam, and you can set up your own whitelist and blacklist of addresses in addition to their filters. I’ve been using their service for many years now, and I love it. If you’re on a lot of mailing lists, though, you’ll probably want to set up a separate account for those that doesn’t go through SpamCop. They make it very easy to report any spam that does get through (on that or any of your other addresses), which serves to improve their filters.