I'm not a teacher, but when I'm in a slump (most of the time the slump occurs when I've played the same pieces for a long time) my teacher immediately come with ideas on new pieces to start with, or talks about what I'm going to play later on.. For one month ago or so, I kind of was in a slump. Then my teacher started talking about a Beethoven sonata that I really really like. Guess who has been practicing a LOT and who isn't in a slump anymore? That's what works for me!