Like Snooze2798 I find that going out of my comfort zone helps me improve. There are a number of courses on- line that I really like as well.
The Craftsy 2013 block of the month taught by Laura Nownes is a free class that has an amazing number of excellent tips for piecing tricky blocks. I took that back in 2013 and my piecing improved quite a bit as a result.
when choosing a course, I like to decide on one area I want to improve, and then look for a course that will help me with that. Right now I'm taking both of the Lisa Calle courses on iquilt because I wanted to improve my Fmq design skills.
Rob