Amavisd-new + spam assassin tuning?

[Olaf Greve]

Hi all,

Perhaps this is not the best place to ask this, but if so, I hope  
someone can quickly point me in the right direction.

The situation:
On my FreeBSD live box I have set up a split Sendmail RX set of  
daemons, such that incoming mail gets tunneled through Amavisd-new +  
ClamAV + SpamAssassin, and then gets delivered to the recipients on  
my machine (unless filtered out by the above programs).
This works fine, however, I'm getting more and more spam, and it  
seems Spam Assassin is not filtering out a lot using its default  
settings.

The question(s):
I'd like to tune Spam Assassin such that it filters out much more  
spam, whilst letting (almost) all proper messages through.  
Thunderbird's spam controls are pretty good at filtering out spam,  
and I was hoping perhaps Spam Assassin can also be taught and/or  
configured to filter out spam very efficiently, whilst letting all  
(or at least almost all) of the real messages through.
Can anyone tell me how to best go about this, and/or (in case this  
isn't the right place to ask this question) where I can best read up  
about achiving this?

Tnx a lot in advance, and cheers!
Olafo