Build a great product and get some early traction. Assemble a top-notch term of co-founders. Understand your space like nobody else. Be good at telling a great story and communicating your vision.

There is really no single "best strategy" that will guarantee success (i.e admission to one of the top accelerators).

That said, over the years, a lot of lessons have been learnt about must-haves in your application to give yourself the best possible shot. For e.g.
[1] http://ycuniverse.com/yc-applying-interviewees
[2] http://venturebeat.com/2013/09/20/how-to-write-a-winning-y-combinator-application/