My biggest jump came when I decided to stop BSing myself.
I used to be a lazy player and make excuses.
I wouldn't use english out of my comfort zone, I'd blame equipment,
and I'd always convince myself the easier shot/english was correct
even though I knew it wasn't.
I chose to make myself do the extra work on shots, hit them the way
they need to be hit, and never settle on leave unless settling is truly the best option.
I got rid of wishful thinking... I stopped telling myself barely blocked balls could pass,
or that I could hold a cut that was too thin, or that I'd send the cue ball towards
a tricky area and just hope it worked out.
Basically it was a mental thing, not a physical thing, though I do work
on my stance and fundamentals.
A longer explanation is here:
http://forums.azbilliards.com/showpost.php?p=793333&postcount=3