Unfortunately, you can't do much to stop spoofing once it starts--or to avoid having spammers harvest your email address in the first place. Irace offers some sarcastic advice on how to make your email address harvest-proof: "Don't do anything interesting [online], and never share your email address with anybody [else]."
Nevertheless, Siems says that adopting some commonsense security practices can reduce your email account's exposure. For instance, she suggests, use your primary email account to communicate only with people you know and trust. If one of those contacts gets infected or compromised, attackers may still harvest and use your email address, but the risk should be much lower.
Also, when sharing an email address with a website or posting information in a public online forum, use a throwaway email account, such as one from Gmail or Hotmail, that you won't mind deleting later on.
These steps amount to hazard mitigation, though. There's simply no fool-proof way to prevent spammers from using your email address in spoofed message headers on spam email.