This trick has already been covered before, but since I forgot about it and you might have forgotten too, it's worth mentioning here: if you use Apple's Mail.app for your email, putting the junk filter back in training mode for a couple of days will significantly improve its performance.

I had noticed that a whole bunch of messages were slipping trough the filter, including many "Check out this document" virus-related ones.

Yesterday I set the application back in training mode (under Preferences > Junk Mail). I manually flagged incoming messages which were not automatically recognized as spam before deleting them for a while.

Now the filter is working much better. I put it back in automatic mode. If you are seeing too much spam in your inbox, it's an experiment worth trying.