I think everybody has some great things to say – I especially enjoyed Carl’s.
I want to make an analogy here. I once attended a seminar by well-known writer Stephen Laws. He said that when he writes he must be careful not to empty his “well“ of ideas each day, if he does, he has nothing to start with on the next day. I think practicing can be like this. If you practice out all your ideas and become exhausted, it will be more difficult to come back to the same piece the next day. If you stop when you’re still excited about it and “could practice longer” you can have a greater desire to come back to it. Sometimes, of course, you just have to grit your teeth and grind through. Similarly, Stephen Laws said when he was uninspired he made himself write his daily “quota” of words anyway – it didn’t matter if he changed them later.
Another thought on practice – listening to a good recording can count as practice. The better you can sing the melody in your head the faster you will learn and the easier everything will be.