I use a webmail account to handle all of my personal e-mail, and over the years the address has gotten around to quite a few spam lists. Fortunately, my account has filters included for sorting e-mail into folders; unfortunately, it's not very sophisticated--I can only filter on "To:", "Cc:", "Subject:" headers, plus the message body, and I have a short limit on the maximum number of filters.
With practice and experience, however, I've found that a few tricks can trap most of the junk e-mail I get on a daily basis.
Filter headers for your own e-mail address. If neither the "To:" nor the "Cc:" field contains my e-mail address, I send the message to the trash. Many "cheap" spam senders put large numbers of addresses in the "Bcc:" field to distribute as quickly as widely as possible. I do subscribe to a handful of e-mail newsletters that use Bcc: as well, but by monitoring my trashed e-mails for a couple of weeks I was able to identify all of these and re-filter them into my inbox.
Unsubscribe actively. Of the e-mails that still get through, many of them are sent by "legitimate" mass e-mail senders. Not all, not even most, but many. When these e-mails have a Web page I can use to unsubscribe from their mailings, I do so. When they only provide an e-mail address to unsubscribe, however, this is invariably either fake or set up to confirm your e-mail address as a valid recipient; when the "unsubscribe" e-mail address is at another webmail provider (yahoo.com or hotmail.com, typically), it's always fake and just there to fool me into thinking it's a "legitimate" mass mailing. But in general, actively unsubscribing from unwanted spam has decreased my daily flow. (Incidentally, this works against telemarketers just as well.)
Filter non-English (or non-your-language-here) characters. Those two tricks worked so well, I didn't need to change it for years--until somehow I got on a few dozen Chinese, Korean and Japanese spam lists. My email is in the "To:" header, but I can't read the message to find the "unsubscribe" link (if there is one). Then I noticed something: while the body of the message was rendered into non-English characters by my browser, the "Subject:" field was not. I looked at a couple day's worth of Asian spam, noticed that the 'Á' and 'À' characters were present in nearly all of them. I added a filter to send all messages with those characters in any header to the trash, and voila--immediate 95% reduction in non-English spam.
None of these tricks has ever successfully eliminated all of my daily spam, but I'm surprised every time I look in my Trash folder at how much it does catch--in a typical day, I might see five e-mails trapped by the system's built-in spam blockers, three get through to my inbox, and fifteen or twenty sent directly to the Trash. Not as good as SpamCop, perhaps, but I'm generally pleased.
As a final note, it's generally agreed that Hotmail and AOL have poor to useless spam filters set up on their own servers, while Yahoo! Mail is much more effective. Still, Yahoo! can't catch everything, and I use the above filters to help trap the junk where Yahoo! can't.