Sawing to a line! It seems to me that even after 15 years and, hundreds of dovetailed drawers, this is the one skill that requires constant attention. I remember taking Woodworking Essentials with Lonnie Bird and we were getting ready to cut our very first dovetails. In order to encourage us to practice Lonnie told the following story:
He Said, "I was teaching an advanced class and one particular student had dovetailed the case of a cabinet flawlessly. It was beautiful and I was a little surprised because this student had not displayed this kind of skill the last time he was in class. I invited the class over to admire this students work and I asked him how he had gotten so much better since the last time he had attended class.
The student explained, 'I made it a point to go to my shop and cut a dovetail joint every day, rain or shine. Over time, my dovetails improved until I could consistently make them fit without a lot of fussing with them.'
"Another student in the class then replied, 'Well, I don't want to do that. I want to build furniture.'"
Lonnie went on to explain, "Some hand tool skills are a little like musicianship. Scales are not the reason people take up music. They want to play beautiful melodies. Nevertheless, all musicians learn to play their scales, if they want to improve. And most musicians warm-up before they take the stage. So, practice sawing to a line. Practice cutting dovetails. And, before you make your first cut on that walnut case, consider warming-up a little first."
I have found this to great advice.