SPAM filter automation

What do you do to help filter SPAM from your corporate mail. I have a Sonic firewall and an Exchange 2000 server. I have been using the Exchange server filter manually adding domains and IP’s that we want blocked and some manual filtering in the Sonic wall for IP’s that are reoccuring frequently but the lists are getting VERY long and it is time consuming adding stuff daily by hand all the time.
Please let me know some good products, services or techniques to reduce our SPAM.

Please bespecific with websites or instruction. All helpful suggestion will get points