As for regressing when practicing, you must keep a cool head. Making mistakes when learning a piece is the number one way to go 'backwards'. If you find yourself getting frustrated, either stop for a while or channel that frustration into a cool-headed determination to practice effectively.
I'm hoping some teachers might have insight into this.When I'm working on a complex piece I'll spend several days in a row struggling through it and feel like I'm regressing or at best, making baby step progress. Then if I take a day or two away from it, when I return it feels like I've progressed by leaps & bounds. Why is that ? What is the process thats going on ? Does the time away clear one's head of the negatives & frustrations ? I can't believe its that simple though....