Attention grabbing title aside, I get this question regularly. I think most people assume the answer will be something like this: brand new XYZ supplement, some prehistoric Indonesian deadlift cycle, Altitude magnifying training masks, and the list could go on. Sure all of these things may help out your Crossfit game but in my experience the number one thing you can do to get better at Crossfit is to make your way to the gym and actually do it with some heart! Set aside at least an hour of your day and do that as many times a week as is smart, and give some real effort. It doesnt matter if it is pre-function or WP+p+. The fanciest movements and best coaching out there will be meaningless without perfect fundamentals or if you dont make it in the door. So always practice your basics (even if you have done an air squat a million times, I bet it still isnt perfect) work hard, and push yourself. I am no great crossfitter by any means but I can usually hold my own, and I did a function wod that completely laid me out not too long ago. That workout did light years more towards making me fitter than if I had done the most badass WP wod at 35%. So keep that in mind, and get to the gym.