The Centre for Technology and Democracy has tried to determine the things that get you spammed the most. Their report seems to indicate posting your email address up on public websites is the worst thing you can do, but there are lots of other ways spammers can get at you. Also, they found that contrary to popular believe unsubscribing to a spammer’s email does not seem to increase your likelihood of receiving further spam.
There are lots more useful details in the report but if you don’t want to read the whole thing, the BBC offers a quick summary.
