Recently, I started adding a tag with the name and id set to "message". I set it to hidden with CSS (display:none). Spam bots see it, fill it in and submit the form. Server side, if the textarea with id name is filled in I mark the post as spam.

Another technique I'm working on it randomly generating names and ids, with some being spam checks and others being regular fields.

This works very well for me, and I've yet to receive any successful spam. However, I get far fewer visitors to my sites :)