I use Crayola Ultra Clean markers or crayons (all colors wash out, but yellow & brown are a bit harder to remove), The trick is not to become totally obsessed with hitting the lines. They're there for guidance & wash out so no one will know if you miss them unless you sharply change directions to get back on your marked line.
Be sure to buy ULTRA CLEAN, not the standard Washables, as only the Ultra Clean come out every single time. I soak in the tub in cold water with a Color Catcher for 20-30 minutes when the quilt is finished & then wash as usual (cold, gentlest cycle, Tide Free, tumble dry low).
My quilting definitely improved when I started marking it because I could focus on stitch formation, rather than splitting my attention with how to form the shape. As I've become more experienced, sometimes I now have the muscle memory to quilt a design without marking it, but I still find it more relaxing to have a line to follow.