I've been keeping with playing keyboard in the morning, even if it's only for 10 or 15 minutes, before I go to work. In the process of learning to play, I'm trying to learn how to learn as well. I saw a quote on a piano list I'm on by Anni Wakerley: "I had my piano lesson this morning and my teacher was saying there are 2 questions she's never known the answer to. The one is 'how do you know when you are in love?' and the other is 'how do you practise the piano?'." So I'm in good company, but I'm trying to find an answer anyway.
First off, there's memorizing the piece you're playing. I've tried it, and I like it. I've already mentioned how much more fun it is to play something that you're making up on the spot, and I think that part of the fun is that you're playing something purely out of your head. Memorizing gets the piece wired into your head, so that playing it is just playing.
Another item a college friend had told me 20 (!) years ago was how different it is to play with your eyes closed. (I think he turned the lights out, but there are way too many toys between my keyboard and the lightswitch; I'd hurt myself on the trip back.) The curious thing is how it changes your mental processes.
Let me put it another way. Seems like there are different tasks going on when you play. One is getting the notes from the paper into your head. Another is getting them from your head to your fingers. A third is getting your fingers on the right keys. Playing the way I had been, reading the notes as I play, involves all three at once. Memorizing the piece (or making it up on the spot) removes the first task, so you can concentrate on the other two. And closing your eyes forces you to concentrate more on the third task.
I would certainly like to be able to sight-read music as I'm playing, and I don't have any desire to play blindfolded, but I do see these two techniques as changing things up a bit, and helping various parts of my playing.
I hope, anyway. I guess we'll see.
Update: Anni said further: "Carol's point was that there are guidelines, but ultimately you have to find the way that works for you. Nothing more uncomfortable than being in a straightjacket of someone else's making."
I couldn't agree more.