Spam
To prevent spam, don't post your email-address at forums/websites. Just like there are 'spiders' and 'bots' (small applications which search the web) for searchengines, to build a database of websites, there are spambots/spiders which crawl the web to find mailaddresses to enter into their spambase. When you register at a forum/website you can use your real address, allways read the privacy statement first. When you need to let other people know where you can be reached, change the format of your address to something like this joeNOSPAM@doe.com. Users will know the need to remove NOSPAM, bots don't.
Never, NEVER, click the 'unsubscribe' button in those mails, you won't be removed from the list but instead you confirm the existence of your address and start receiving more spam.
To finish the 'spam-part' when signing up for a free website, a sweepstake etc. Read the privacy policy and never, when the option is available, select what your 'interests' are when asked. When you do you will receive targeted spam, spam about the subjects you chose.
You can report spam here: spamcop