My trash folder used to hold about 2,000 spam (and non-spam) messages. Any mail older than 7 days is automatically deleted. Most of what was there never got to my email client, because I use bogofilter to do bayesian spam filtering.
That worked well on its own until I started getting _tons_ of spam. I wrote a bunch of scripts to identify the offending IPs and compile them into my own DNSBL (DNS Block List). It is publicly available at dnsbl.jdrowell.com. That's not a homepage, but a domain for the reverse IP lookups.
Since I started using this DNSBL, my trash folder trimmed down to about 200 messages (for the week). That includes my legitimate email (which I read and then delete). Not bad :) It also unloads my mail server, and, most importantly, makes spammers really angry. And poor. And suicidal (I wish).
The current count for dnsbl.jdrowell.com is about 70,000 IPs. I don't add blocks, only single IPs. I don't remove IPs unless I feel like it. I don't recommend that anyone use this DNSBL to actually block messages, but instead to flag spam as part of some greater process, such as using SpamAssassin or another similar tool.
That's about it. At a rate of about 2,000 new IPs every day (boy do I get spammed!), I'll probably have over 100,000 spam sources identified by the time you read this! Bring on the zombie botnets!
Update: The database has over 1 million IPs now. Scary.