For me, it was getting more behind the bird. Smashes are steeper and I'm actually looking where I'm hitting lol
I find that if my heels do not touch the ground (assuming I am not jumping), the accuracy of my smash improves.
After seeing some beginners play yesterday, I was reminded of how I started out. I'm going to have to add "switching from panhandle to basic grip". Panhandlers seem to use twice the effort for half the power and can't return anything once it's gone past them.
A relaxed yet focused mind and body plays the best. Found this out from injuries and from being pushed to limits. My mind and body will automatically go into a "I give up" mode, but ironically that makes me better through very efficient use of techniques. How I accomplish it is unknown to me. My brain and body does it for me. Subconscious I guess.
Playing with finger power and wrist. I also found out the best possible returns to specific shots by watching the pros.