For me it's definitely a "learn by doing" thing. My first FMQ end product makes me cringe just a little bit, but I gave it to my sister, who looks at it with an un-critical eye and loves it. I made the rule for myself that I would not rip out my work -- just keep going and try to make the next part look better.
I find it helps to practice regularly -- 15 minutes every day, just to make it feel natural and intuitive. I have scraps I piece into improv pillow covers (24 inches square) or table runners (18 inches by 70) and I will practice on those every day. The end results are a mixed bag, but they work really well as picnic table covers or pillows for pets. Some recent practice pieces are so nice that my niece asked to have a quilt made of them. I use the practice pieces to try out new stitches, trial threads, think through the approach to a design I intend to do on a real quilt -- they are really my key to success.