In this post, Donnie mentioned the use of various spam filters and IMAP proxies.
I don’t know about other people, but most of the spam I receive is in character sets that I can’t even read. So, it only takes one simple procmail rule to filter them all out:
:0 * Content-Type:.*(big5|gb2312|euc-kr|ks_c_5601-1987).* /dev/null
It makes sense to put this sort of thing before your spam filters, as it will use nowhere near the resources.