pff, this is not easy, i'd say you have two options. now please correct me if i'm wrong, i have no experience with this whatsoever!
first thing you could to to evade antivirus is make sure the code is different so it will not match the signature of the antivirus. You can do this by adding characters that may not be used. you can use the following parameter for this: -b The list of characters to avoid: 'x00xff'
another option would be to obfuscate the code or to attach the code to another executable, but i dont have any examples on that.
you probably already seen this one? http://www.offensive-security.com/metasploit-unleashed/Antivirus_Bypass