Below are various lessons written by Bill Mattocks on how to identify the source of spam:

• Spam-tracking 101 (for newbies)
• Spam-tracking 102 (the many uses of DejaNews)
• Spam-tracking 103 (the whois tool)
• Spam-tracking 104 (a case study: spammer unmasked)

If you are lazy then you can also use the Spamcop service, where you can paste the entire email and it will automatically find the spammer's ISP and web hosting company and send out complaints.

Now that you've learned how to track the source of the spam, here's my lesson on how to respond.

• How to complain about spam

After you have read the above, here are some spam tracking tools. I personally use Sam Spade, Spam hunter on a frequent basis.

• Steve Atkins Spam tools

Here are some other links on reading Email headers

• Reading Email Headers - excellent article on understanding email headers
  
• Afterburner Spam Header Lesson
• Dealing With Junk Email
• Tracing the Source of Junk Email