The Center for Democracy and Technology has run a study that should help you. It created more than 250 email addresses and waded through 8,842 pieces of spam to find the answers. Some of them are quite surprising.
The way to get the most spam is to publish your email address on a popular web site. You might be OK if your email address only appears somewhere very obscure but even then you can't be certain. Spammers use special programs to 'harvest' email addresses from web pages.
The good news is that you can kill off the quantity of spam you receive pretty quickly by removing the email address from the page. Or changing your email address to be human readable but not machine readable, for example "myname at mydomain dot com" will throw the spammers' harvesting programs off the scent.
One of the bigger surprises was that, contrary to popular belief, USENET is not as big a target as web sites for the harvesters. You're in for at least some spam if you include your real address in the header but relatively safe if you put your address in the body of a post. Once again, making the address only readable to humans killed all the spam.
According to the report, most companies honour their privacy policies. The two major exceptions being online gambling and porn sites, quelle surprise. However, some companies took quite a while to stop sending emails once they were asked.
The only disappointment we have with the report is that it didn't test what happens when you request removal from a spam list, as offered at the bottom of many pieces of junk mail. Popular consent is that the spammers then know they've got a live one and so redouble their efforts. It would have been nice to see some confirmation.
Either way, it's a good report and well done to Center for Democracy and Technology for having done it. You can read the full report here. µ