Next, notice your mistakes and why you made them. Did you make mistakes you weren't expecting, or were they in places you expected, places that had been giving you difficulty? Just because you made mistakes in places you were having trouble with doesn't mean that such mistakes are inevitable, rather that you should start to learn how much practice it will take to overcome a known difficulty. If some mistakes were unexpected, frequently this comes from your mind wandering as you perform your song; practicing concentration while performing is actually quite difficult - but it can be done. If you were nervous, now did much it effect your performance, to what degree? In each of these areas, I'm saying you can learn to gage how long and how thoroughly you need to master a song and its pieces to successfully perform it. You can use this information to prepare next time (in both time and method). As an example, most people speed up their songs when they're nervous. If you did this, once you know it, you can remember that next time and try to keep the right tempo as you perform. Also , because of this examination you may come up with new areas you should be working on that the performance brought up.
Complete a song each month; 2 months if complicated. Choose difficulty of pieces accordingly. Pieces offer new abilities so get through many.