there are a few good methods out there. which one you use depends on your system and costs.
option 1: if you are running outlook you can try spambayes (
http://sourceforge.net/projects/spambayes). it learns what you think is spam as you use it - free - i use this at home.
option 2: run a comersial product on your system like McAfee's Spam Killer - not free
option 3: if this is an office and you run your own e-mail server try MailMarshal SMTP. I use this one and turned an average 400 spam a night (5pm to 9am) into 1 or 2.
option 4: use an external spam filtering company and have your mail directed through them before you get it. simular results to option 3.
option 5 use an external company that requires each user that sends you e-mail to verify themselves (only on the first e-mail). this is very effective against spammers that use fake reply addresses (as most do).
lastly, running a mac is not an anti spam soloution as any address can be harvested. the mac will help against spywhere that is looking for addresses on your system but that's not how all addresses are found. if you are useing a mac specific e-mail provider then they 'may' be running anti-spam filters and that is why the mac user 'may' see less spam. if you where running a PC and was on 100 spam lists and then you installed a mac, you are still on the spam lists and will still recive the spam messages.
Just asked a mac head and he said that the mac mail program has learning abilities just like option 1 above and that it learns what is spam and what is not.