I thought up this idea last year and it has saved me from 2 viruses so far. A co-worker and a friend of the wife thought my idea was dumb at first, and then they were hit with the dbs virus in October, and that latest virus in January. They laugh no more.
Whenever I forward an attatchment, I always put a personal note, something like:
Hey I thought this was pretty funny,
-Justin
or:
This joke is right on the money, especially for Paul
-Justin
this way everyone knows that when they get my forward, it's safe to open. I sent an email to everyone in my book telling them to do the same. When I get an email that does not have a personalized message so that I can authenticate that it was not "auto-forwarded" as viruses always do, I send a reply asking did they send this, and if so, they need to say so or it won't get opened. If they don't authenticate, I just dump it.
I suppose if everyone did this, viruses wouldn't get too far. I suppose until the virus makers started putting in a program that "auto-writes" happy messages, but until then, this looks (to me at least) pretty foolproof.