Google's Gmail filter is based on the well-known Postini filter widely used by corporations.
It's  filter that takes into account many different variables to determine whether mail is spam or real mail.
It is normally a commercial service, but by using Gmail you effectively get Postini for free.

With this method you won't be using this Gmail account to replace your normal e-mail address but rather as a transit stop.
The idea is to funnel the e-mail from your normal account into Gmail and then pick it up from Gmail using your normal e-mail client after it has been Postini spam filtered at Gmail.

The trick to Using Googles webmail as your spam filter:

If you already use Gmail and have an e-mail account with POP3 access enabled for that account, you can skip straight to Section 2.
If you don't use Gmail, set up an account. It's simple, it's quick, and it's free.
You can create multiple email address's / accounts on google if desired.

Click Here to go to Googles Gmail Service

Section 2:

Step 1:
Once you have created your Gmail account, enable it for POP3 access by following the steps described on Google's site.
This is an important step because you'll use POP3 to retrieve your e-mail from Gmail.

Set Gmail to retrieve messages from your POP3 account via the service's Mail Fetcher settings.
Set up Gmail's Mail Fetcher to download messages from your normal e-mail account to your Gmail account.
Google offers step-by-step instructions.
Googles Mail Fetcher works only with e-mail accounts that use POP3.
If your normal e-mail account doesn't have POP3 access, you'll have to use the settings in your e-mail providers control panel to forward your POP3 messages to your Gmail account.

Step 2:
Next change the setting in your POP3 Client for the email account your filtering through gmail, so it doesn't download mail directly from that account when you receive mail.
Uncheck / disable the option in the account settings for your POP3 Client for "Include this account when recieving mail or synchronizing" for the account your now funneling through Google, so you wont download mail from the original source that is un-filtered. You should not delete this account in your POP3 Client because you will still use it as your default email address when sending email. Sending email does not require spam filtering. You will still send through your original email account, not gmail.

Step 3:
Set up your POP3 Client to download mail from the gmail account you set up for Postini spam filtering.
Google provides excellent POP3 configuration instructions for Outlook, Outlook Express, and Windows Mail.
Remember the Gmail account must be configured for POP3 access.

Configuring your email client:

Your User Name and Password
Remember my password checked

Incoming Mail Server:
pop.gmail.com
Outgoing Mail Server:
smtp.gmail.com
Server Requires Authentication checked
Use Same Settings as my incoming mail service checked

POP3 Advanced Settings:
Outgoing Mail SMTP 465
Server requires a secure connection (SSL) checked

POP3 Advanced Settings:
Incoming Mail POP3 995
Server requires a secure connection (SSL) checked

Step 4: Test your spam filter
Mail sent to your original account will be downloaded by gmail every few minutes, and filtered by Postini.
The mail that makes it through the spam filter (your real mail) will end up in Gmail's inbox, where it will wait for you to download it with your POP3 client.