1. Be suspicious of everything. I hate to say be suspicious but hackers are very good at hiding viruses.
2. Set your email properties to NOT automatically open mail. Many email programs are set to open mail when its highlighted. Turn this off.
3. Delete spam and other suspicious mail before opening any mail at all.
4. Develop a feel for what mail is spam and/or may contain a virus. Yes, you may delete a legitimate email. But that is far better than the hassles of eradicating a virus. Along this vein, when sending email help the recipient by using the subject line.
5. And keep the virus protection current. This means almost daily updates.