There are no legitimate "viral strategies." Having a "strategy" implies there are "best practices" you can follow that will virtually guarantee you success.
But viral is not something you do
on purpose. It's something that happens
because something you did for some other purpose caught the public imagination.
Create compelling, interesting, entertaining content that promotes your product or service. If you're lucky, somebody will like it, and they'll pass it along to their friends... who will in turn pass it along to their
friends... who will, in turn, pass it along to their
friends... and you'll have a viral hit.
On the other hand, you may create additional content that's every bit as interesting, entertaining and original as the first, but it goes nowhere. It just depends on what catches the popular imagination at the time.
There's no way you can guarantee it will happen. There's no way you can make
it happen. You just do the best you can and hope it works.